Columbia  ©mbergttp 
m  tfje  £ity  of  Jgeto  gorfc 

COLLEGE  OF  PHYSICIANS 
AND   SURGEONS 


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NOTES 


ON   THE 


PRACTICAL    COURSE 


IN 


Normal   Histology 


GIVEN    IN    THE 


Laboratory  of  the  Alumni  Association 

OF   THE 

COLLEGE  OF  PHYSICIANS  AND  SURGEONS 

NEW  YORK   CITY 


BY 

T.    MITCHELL    PRUDDEN,    M.D. 


NEW   YORK 
TROWS   PRINTING   AND   BOOKBINDING   CO., 

205-213  East  12TH  Street 
1879 


PREFACE. 


The  full  course  for  classes  in  Normal  Histology  in  this  laboratory 
comprises  forty  lessons,  of  from  one  and  a  half  to  two  hours  each. 
Students  are  furnished  with  microscopes,  reagents,  and  all  necessary 
apparatus,  with  the  exception  of  razors,  slides,  cover-glasses,  and  boxes 
for  the  preservation  of  specimens.  So  little  time  is  usually  at  the  dis- 
posal of  medical  students  for  collateral  reading,  and  so  necessary  is  it 
to  occupy  as  little  of  the  laboratory  time  as  possible  in  oral  descrip- 
tions of  tissues  and  methods,  that  these  notes  have  been  prepared 
with  the  expectation  that  students  will  anticipate  each  lesson  by  read- 
ing beforehand  the  brief  section  devoted  to  its  theme,  and  thus  be 
ready  to  commence  the  practical  work  of  the  lesson  hour  without  loss 
of  time. 

It  is  not  to  be  expected  that  epitomized  descriptions  of  structures 
as  elaborate  as  are  many  of  those  with  which  we  have  to  deal  in  his- 
tology, will  be  in  all  cases  perfectly  clear  and  intelligible  without  the 
aid  of  figures ;  but  the  actual  specimens  prepared,  and  the  sketches 
from  them  which  are  made  in  the  laboratory  by  the  students  them- 
selves, will  make  good,  it  is  hoped,  the  lack  of  illustration  in  the  text. 

There  are  many  points  in  this  as  in  every  developing  science  which 
are  still  unsettled — opinion  in  regard  to  them  changing  or  being  modi- 
fied as  new  facts  and  investigations  are  recorded.  These  have  been 
treated,  for  the  most  part,  very  briefly  in  the  text,  it  being  left  for  the 
supplementary  oral  instruction  to  enlarge  upon  and  explain  them  as 
the  light  thrown  upon  each  by  new  researches  may  seem  to  require. 

T.  M.  P. 

Laboratory  of  the  Alumni  Association,  College  of  Physicians 
and  Surgeons,  New  York,  Sept.,  1879. 


INTRODUCTION. 


GttJNJiRAL   METHODS   FOR   PRESERVING  TISSUES  AND  PREPAR- 
ING THEM  FOR  STUDY. 

Animal  tissues  must  conform  to  certain  physical  conditions  before  they 
can  be  subjected  to  a  satisfactory  microscopical  examination.  Por- 
tions of  them  subjected  to  study  must  be  sufficiently  thin  to  allow  the 
light  to  pass  readily  through  them,  and  transparent  enough  to  permit 
the  determination  of  the  form,  character,  and  relations  of  their  struc- 
tural elements.  At  the  same  time  the  refractive  power  of  the  different 
elements  should  not  be  too  nearly  alike,  since  upon  differences  in  this 
respect  the  form  and  characters  which  microscopical  objects  present 
to  the  eye  are  largely  dependent ;  or,  in  case  they  are  so,  the  differ- 
ent elements  must  be  rendered  visible  by  staining  them  with  coloring 
agents.  Certain  tissues  naturally  undergo  rapid  changes  of  structure 
after  death ;  these  are  to  be  prevented  by  the  application  of  preserva- 
tive agents.  Some  are  too  soft  to  permit  the  preparation  of  thin  sec- 
tions and  must  be  hardened ;  others  are  too  hard  and  must  be  softened. 
In  some  specimens  one,  in  others  another  structural  feature,  is  to  be 
brought  into  prominence.  All  of  these  indications  in  the  histological 
technique  are  to  be  met  in  such  a  way  as  to  leave  the  structures  under 
investigation  in  as  natural  a  form  as  possible.  Finally,  specimens 
suitably  prepared  for  examination  are,  in  many  cases,  to  be  rendered 
permanent  for  future  reference  and  study. 

We  will  now  consider  briefly  some  of  the  methods  by  which  these 
indications  may  be  fulfilled.  Most  histological  specimens  are  laid 
either  with  or  without  some  enclosing  fluid  medium,  on  a  glass  plate, 
and  covered  with  a  very  thin  slip  of  glass,  before  being  brought  under 
the  instrument.  One  of  the  simplest  methods  of  studying  tissues  is  to 
place  them,  when  quite  fresh,  and  after  they  are  reduced  to  a  condition 
of  suitable  tenuity,  on  a  slide  with  some  fluid  which  alters  their  physi- 
cal condition  but  little  or  not  at  all,  or  at  least  very  slowly,  and  ex- 
amine them  at  once.  Such  fluids  are  called  indifferent  fluids ;  and 
among  the  best  and  most  commonly  employed  are  the  aqueous  humor, 
blood-serum,  amniotic  fluid.  These  organic  fluids,  however,  although 
well  suited  for  this  purpose,  are  not  always  readily  obtained,  and  are 
moreover  liable  to  undergo  more  or  less  rapid  decomposition ;  and 
since  for  most  purposes  a  dilute  solution  of  common  salt,  one-half  to 
three-quarters  per  cent.,  answers  very  well,  we  shall  generally  employ 


6  NORMAL   HISTOLOGY. 

this  when,  in  the  following  lessons,  we  have  occasion  to  use  an  indif- 
ferent fluid  in  the  study  of  fresh  tissues. 

The  examination  of  fresh  tissues  is  very  important,  not  only  because 
it  enables  us  to  follow  the  vital  phenomena  in  certain  'elements,  but 
because  we  are  able  by  comparison  to  determine  the  amount  of  change 
which  tissues  undergo  when  prepared  by  more  elaborate  methods. 
Still  this  simple  mode  of  examination  is  in  many  respects  unsatisfac- 
tory. In  the  first  place,  it  is  not  always  easy  to  procure  fresh  tissues 
for'  every  observation,  and  even  in  the  indifferent  fluids  the  tissues 
sooner  or  later  undergo  very  considerable  structural  alterations,  so  that 
they  cannot  be  permanently  preserved.  Again,  fresh  tissues  are  fre- 
quently not  sufficiently  hard  and  firm  to  allow  the  necessary  prepara- 
tion of  specimens.  A  still  more  important  difficulty  which  this  method 
presents  is  the  lack  of  clearness  in  the  details  of  structure  of  fresh 
tissues.  A  very  considerable  proportion  of  the  fresh  animal  tissues  are 
nearly  transparent,  in  thin  pieces,  and  their  structural  elements  possess 
so  nearly  the  same  refractive  power,  that  we  see  through  them,  but  do 
not  see  them  :  or,  if  we  do  see  them  indistinctly,  it  is  not  generally  with 
that  definiteness  which  our  purposes  demand.  Now,  these  difficulties 
are  usually  met  by  the  employment  of  agents  which  harden  and  pre- 
serve the  tissues  and  at  the  same  time  render  the  details  of  their  struc- 
ture visible,  by  changing  the  refractive  power  of  one  or  other  of  their 
elements ;  or  we  employ,  as  above  indicated,  certain  coloring  agents, 
which,  being  taken  up  with  different  degrees  of  avidity  by  different 
parts,  permit  the  recognition  of  details  by  differences  in  color ;  or, 
such  agents  are  used  as  both  harden  and  stain  at  once ;  or,  finally, 
which  is  the  most  common  method,  we  employ  two  or  more  of  the 
different  classes  of  agents  one  after  the  other.  We  shall  consider 
here  only  some  of  the  most  commonly  employed  of  the  hardening, 
preservative  and  coloring  agents. 

Alcohol  is  one  of  the  most  valuable  of  the  preservative  and  harden- 
ing agents.  It  causes  a  considerable  shrinkage  of  the  tissues  by  the 
withdrawal  of  water  from  them,  and,  like  many  of  the  preservative 
agents,  causes  a  precipitation  of  certain  of  their  albuminoid  constitu- 
ents, thus  diminishing  their  transparency. 

Alcohol  is,  in  general,  to  be  used  at  first  diluted  -with  one-third 
water,  and  after  the  bits  of  tissue  have  lain  for  twenty-four  hours  in 
this  they  are  transferred  to  commercial  alcohol  (eighty-five  per  cent.), 
in  which  they  may  be  preserved  indefinitely,  losing  in  time,  how- 
ever, somewhat  of  the  first  clearness  and  naturalness  of  structural 
detail. 

Bits  of  tissue  to  be  preserved  in  alcohol,  as  in  other  hardening 
agents,  should  be  quite  small — not  larger,  as  a  rule,  than  i  or  2  cms. 
on  a  side — and  the  quantity  of  fluid  should  be  abundant.  Certain 
structures  are  best  preserved  by  plunging  them  at  once  into  strong 
alcohol. 

Chromic  acid  in  solution  and  solutions  of  potassium  and  ammo- 
nium bichromate  are  very  frequently  employed  to  preserve  and  harden 
tissue,  and  many  structures  are  more  perfectly  preserved  in  these  fluids 
than  in  alcohol.     The  hardening  process  proceeds  more  slowly  in  the 


INTRODUCTION.  7 

chromic  solutions  than  in  alcohol,  and  the  structures  do  not  shrink  as 
much.  The  tissue  seems  to  be  hardened  and  preserved  by  a  slow 
process  somewhat  analogous  to  that  of  tanning. 

It  is  a  common  practice  to  commence  the  hardening  process  with 
one  of  the  chromic  fluids,  and  complete  it  with  alcohol. 

Pure  chromic  acid  is  usually  employed  in  solution  of  from  one- 
sixth  to  one-half  per  cent.  Potassium  and  ammonium  bichromate  are 
usually  used  in  2  per  cent,  solutions.  A  very  valuable  and  much  em- 
ployed preservative  solution  is  the  so-called  Midler's  Fluid,  consist- 
ing of — 

Sodium  Sulphate I 

Potassium  Bichromate 2 

Water 100 


The  ingredients  are  simply  dissolved  in  water  and  the  solution  fil- 
tered. Chromic  acid  dissolves  the  lime  salts  in  bone,  and  is  often  used 
to  soften  them  in  preparation  for  section-cutting.  Tissues  which  con- 
tain fat  are  usually  better  preserved  in  the  chromic  fluids  than  in  alco- 
hol, since  the  fat  is  readily  dissolved  by  the  latter. 

Picric  Acid. — This  agent  hardens  tissues,  preserving  in  many 
cases  their  structural  features  most  perfectly,  and  at  the  same  time 
staining  them  intensely  yellow.  It  is  generally  necessary  to  complete 
the  hardening  process  with  alcohol. 

Picric  acid  is  one  of  the  best  agents  for  the  decalcification  of  bone, 
although  it  acts  very  slowly.  It  is  commonly  employed  in  cold,  satu- 
rated, aqueous  solution. 

Osmic  Acid. — This  substance  has  the  power,  in  dilute  solutions, 
of  fixing  and  hardening  the  tissue  elements  in  a  nearly  normal  form, 
and  is  one  of  the  most  valuable  of  this  class  of  agents.  It  gives  tis- 
sues a  gray  or  brown  appearance,  and  stains  fat  and  certain  allied 
substances  deep  black.  It  is  a  very  expensive  substance,  and  hence 
its  use  is  at  present  somewhat  limited.  It  is  generally  employed  in  a 
one  per  cent,  aqueous  solution,  and  the  tissues  should  be  quite  fresh 
when  immersed  in  it,  and,  as  a  rule,  should  remain  for  twenty-four 
hours.  Specimens  hardened  in  osmic  acid,  although  very  perfect  at 
first,  commonly  become  quite  granular  and  black  after  a  time,  and 
nearly  worthless. 

The  preservative  fluids  are  sometimes  brought  into  more  perfect 
and  immediate  contact  with  the  tissue  elements  by  injecting  them 
into  the  blood-vessels  of  the  part  before  cutting  them  in  pieces  and 
immersing  them  in  the  fluids.  This  method  is  of  special  value  when 
alcohol  is  used  for  hardening  small  organs  like  the  kidney. 

Indications  as  to  which  of  these  agents  are  best  adapted  for  the 
preservation  of  different'  tissues,  and  the  more  exact  details  of  the 
methods  of  employing  them,  will  be  given  as  we  proceed  with  our 
practical  study. 

Hematoxylin  is  one  of  the  most  generally  useful  of  the  staining 
agents.  It  has  the  power  of  staining  certain  parts,  as  the  nuclei  of 
cells,  deeply,  while  other  parts  are  much  less  or  not  at  all  stained.     It 


8  NORMAL   HISTOLOGY. 

can  be  employed  for  staining  tissues  which  have  been  hardened  by 
any  of  the  above  agents. 

The  following  is  Prof.  Delafield's  method  of  preparing  the  solu- 
tion : 

To  make  200  c.c.  of  the  solution,  take  150  c.c.  of  saturated  solu- 
tion of  ammonia  alum  ;  prepare  a  saturated  solution  of  haematoxylin 
in  absolute  alcohol,  and  add  4  c.c.  of  it  to  the  alum  solution.  This  at 
first  produces  a  violet,  or  sometimes  a  dirty  red  color,  which,  on  ex- 
posure to  the  light  in  an  open  vessel,  usually  assumes  in  a  few  days  a 
deep  violet  color.  If  the  color  does  not  become  deep  enough,  a  few 
drops  more  of  the  hematoxylin  solution  are  added  and  the  fluid  exposed 
anew  to  the  light.  After  standing  for  at  least  a  week,  and  when  the 
desired  color  is  obtained,  the  solution  is  filtered  and  25  c.c.  each  of 
glycerine  and  wood  naphtha  are  added.  Such  a  solution  is  to  be  diluted 
with  several  times  its  bulk  of  water  before  using,  the  exact  amount 
of  dilution  depending  upon  the  rapidity  with  which  we  wish  the  speci- 
men to  be  stained.  As  a  rule,  slow  staining  with  a  dilute  solution 
gives  the  best  results,  and  is  less  likely  to  cause  shrinkage  of  the  spe- 
cimen. In  staining,  bits  of  tissue  are  simply  placed  in  a  small  dish  of 
the  solution,  so  that  they  are  bathed  on  all  sides  by  it  and  allowed  to 
remain  until  sufficiently  colored.  The  time  required  will  depend,  of 
course,  upon  the  strength  of  the  solution,  and  also  to  a  considerable 
degree  upon  the  character  and  previous  preparation  of  the  tissue. 
The  excess  of  coloring  fluid  is  to  be  thoroughly  washed  out  of  the 
specimen  by  water  before  mounting  and  studying. 

Carmine. — This  is  employed  in  the  same  manner  as  hematoxylin, 
and  like  it  stains  different  tissue  elements  with  different  degrees  of  in- 
tensity. Tissues  preserved  in  chromic  acid  solutions  do  not  stain  as 
readily  in  carmine  as  in  haematoxylin. 

Fre/s  method  for  its  preparation  is  the  following  :  take  of  powdered 
carmine  0.30  grm.,  and  add  a  sufficient  quantity  of  aqua  ammonias  to 
dissolve  it,  mix  with  30  c.c.  of  distilled  water,  filter,  and  add  glycerine 
30  grms.,  alcohol  4  grms. 

Eosin. — This  substance  stains  tissues  somewhat  more  uniformly 
than  those  just  mentioned,  and  is  especially  valuable  when  used  in 
connection  with  other  coloring  agents,  such  as  haematoxylin,  which 
stain  the  cell  nuclei  more  deeply,  since  by  this  method  of  double 
staining  we  have  certain  structural  elements  exhibiting  one  color, 
others  another.  Eosin  may  be  conveniently  used  either  in  aqueous  or 
alcoholic  solutions  of  1  to  100. 

Picro-carminate  of  Ammonia  or  Pier o-car mine. — This  substance  is 
for  many  purposes  superior  to  the  simple  carmine.  It  usually  stains 
more  rapidly  than  the  latter,  and  we  can  obtain  with  it  at  once  the 
yellow  color  from  the  picric  acid  in  certain  elements,  while  others  are 
stained  red  by  the  carmine. 

It  is  prepared  by  adding  to  a  saturated  solution  of  picric  acid  a 
strong  ammoniacal  solution  of  carmine  to  saturation,  evaporating  the 
mixture  to  one-fifth  its  bulk,  allowing  to  cool,  filtering  from  the  de- 
posit, and  evaporating  the  filtrate  to  dryness  over  a  water-bath.  The 
picro-carmine  is  left  in  the  form  of  a  crystalline  ochre-red  powder. 


INTRODUCTION.  9 

This  powder  should  be  dissolved  in  water,  for  use,  in  the  proportion 
of  t  to  100. 

Certain  fluid  tissues,  such  as  blood,  lymph,  etc.,  are  fitted  for 
study,  either  fresh  or  after  suitable  preservation,  when  they  are  spread 
out  in  thin  layers  and  covered.  Certain  tissues  occur  in  the  form  of 
membranes  of  sufficient  thinness  to  admit  of  study  without  other  ma- 
nipulation than  spreading  them  out  smoothly  on  a  slide.  In  other 
cases  we  have  recourse  to  the  dissociation  of  tissues  by  needles,  the 
structure  being  carefully  separated  into  parts  sufficiently  minute  for 
microscopical  examination.  The  operation  of  dissociation  with  needles, 
or  picking,  or  teasing,  as  it  is  commonly  called,  will  vary  in  its  details, 
depending  upon  the  nature  of  the  tissue  and  the  special  feature  which 
we  wish  to  study.  In  many  cases  we  wish  to  study  the  structural  ele- 
ments of  a  tissue  in  their  normal  relations  to  one  another,  and  in  parts 
which  are  too  thick  to  permit  a  direct  observation.  In  such  cases  we 
have  recourse  to  thin  sections  cut  from  the  tissues  by  a  sharp  knife  or 
razor — the  tissue,  if  not  sufficiently  hard  naturally,  being  hardened  by 
one  or  other  of  the  above  described  methods.  The  razor  employed 
for  this  purpose  should  have  a  thin  blade,  perfectly  flat  on  the  lower 
side,  as  it  is  held  horizontally  in  the  hand  for  cutting,  and  somewhat 
concave  on  the  upper  side,  so  that  a  small  quantity  of  fluid  will  lie 
upon  it. 

It  is  often  desirable,  in  studying  the  distribution  of  the  blood-  or 
lymph-vessels,  to  fill  them  by  injection  with  some  colored  substance 
by  means  of  which  their  ramifications  may  be  readily  recognized.  One 
of  the  most  convenient  and  generally  employed  injecting  materials  is 
a  solution  of  gelatine  colored  with  Prussian  blue.  This  may  be  pre- 
pared as  follows  :  dissolve  4  grms.  of  gelatine  in  60  c.c.  of  pure  water 
on  a  water-bath ;  divide  the  solution  into  two  portions  :  to  one  portion 
add  4  c.c.  of  a  saturated  solution  of  sulphate  of  iron  (green  vitriol), 
stirring  constantly;  to  the  other  add  first  8  c.c.  saturated  solution  of 
ferrocyanide  of  potassium,  and  then  8  c.c.  saturated  solution  of  oxalic 
acid ;  these  two  portions  are  now  to  be  slowly  mixed  with  constant 
stirring,  and  then  heated  up  to  about  the  boiling-point  of  water.  The 
solution  is  now  filtered  hot  through  flannel  and  is  ready  for  use.  An 
animal  injected  with  this  solution  must  of  course  be  kept  warm  during 
the  injection,  as  must  all  the  utensils  employed,  so  that  the  gelatine 
may  not  harden  and  stop  the  vessels.  The  details  of  the  process  of 
injection  will  be  given  in  the  course  of  our  practical  study. 

It  often  occurs  that  a  bit  of  tissue  from  which  we  wish  to  prepare 
a  section  is  too  small  and  delicate  to  be  held  in  the  fingers  ;  in  such 
cases  the  object  may  be  placed  between  two  bits  of  some  hardened 
tissue,  such  as  liver,  or  between  two  bits  of  soft  cork,  and  thus  held 
while  the  section  is  made.  Or  such  a  specimen  may  be  embedded  in 
a  mixture  of  white  wax  and  paraffine,  equal  parts,  melted  together 
with  addition  of  a  sufficient  quantity  of  olive  oil  to  give  the  mass  the 
proper  consistency  for  cutting  when  cold.  The  details  of  the  methods 
of  teasing,  embedding,  and  section-cutting,  will  be  learned  in  the 
course  of  our  practical  studies. 

Sections,  bits  of  dissociated  tissue,  membranes,  etc.,  having  been 


10  NORMAL  HISTOLOGY. 

dul)T  prepared,  they  are  to  be  mounted  on  a  slide  for  study.  The 
choice  of  the  fluid  to  be  employed  for  this  purpose  will  depend  upon 
the  nature  of  the  specimen,  the  mode  of  preparation  to  which  it  has 
been  subjected,  and  the  structural  features  which  we  wish  especially 
to  demonstrate.  One  of  the  fluids  most  frequently  employed  is  glyce- 
rine. Many  of  the  preservative  and  hardening  agents,  such  as  alcohol, 
precipitate,  as  above  remarked,  certain  albuminoid  substances  in  the 
tissues,  in  the  form  of  small,  strongly  refractive  particles,  thus  render- 
ing them  more  or  less  opaque,*  or  at  least  translucent.  Now,  glycerine 
possesses  the  power  of  penetrating  many  such  tissues,  and  since,  as  a 
rule,  its  index  of  refraction  is  much  more  nearly  like  that  of  the  albu- 
minous particles  than  is  the  refractive  index  of  the  substance  lying 
between  them,  when  the  tissue  becomes  soaked  with  glycerine  the 
light  passes  more  directly  through  and  the  tissue  becomes  more  trans- 
parent. 

Many  specimens,  further,  preserve  their  structural  features  for  a 
long  time  very  perfectly  in  glycerine.  The  specimens  are  either 
soaked  until  they  become  transparent,  in  a  small  dish  of  glycerine, 
and  then  transferred  to  a  slide  and  covered  with  a  drop  of  the  same  ; 
or  they  may  be  put  at  once  into  a  drop  upon  the  slide  and  covered. 
The  preparations  may  be  made  permanent  by  painting  a  rim  of  some 
kind  of  varnish — such  as  asphalt  varnish — around  the  edge  of  the 
cover-glass. 

The  strongly  refractive  power  of  glycerine,  although  of  value  in 
rendering  tissues  transparent  in  the  way  just  described,  is,  however,  in 
some  cases  prejudicial  to  our  aims,  because  it  makes  them  too  trans- 
parent, its  refractive  power  being  so  nearly  like  that  of  the  tissue- 
elements  themselves  that  they  remain  nearly,  invisible,  or  their  more 
delicate  structure  is  concealed.  For  we  find  within  certain  limits  that 
the  greater  the  difference  between  the  refractive  power  of  an  object 
and  that  of  the  fluid  in  which  it  lies,  the  more  distinct  will  be  the  out- 
lines of  the  object.  We  have,  then,  in  using  glycerine  as  a  mounting 
medium  which  renders  tissues  transparent,  to  guard  against  its  too 
excessive  action  ;  and  this  may  be  done  by  mixing  with  it,  in  propor- 
tions varying  with  the  tissue  under  investigation,  some  less  refractive 
substance  such  as  water.  For  permanent  preservation,  however,  most 
tissues  are  to  be  put  into  pure  glycerine. 

Ca?iada  balsam  is  for  many  tissues  a  most  excellent  mounting 
medium.  It  possesses,  to  a  still  greater  degree  than  glycerine,  the 
power  of  rendering  tissues  transparent,  concealing  proportionately,  in 
the  manner  above  described,  certain  of  their  minute  structural  features. 
This  difficulty  can,  however,  be  to  a  considerable  extent  obviated  with 
balsam,  as  with  glycerine,  by  the  judicious  use  of  coloring  agents. 
The  most   convenient  way  of  using  Canada  balsam  is  in  solution  in 

*  A  transparent  object  becomes  more  or  less  opaque  or  translucent  when  in  any- 
way it  is  made  to  enclose  a  multitude  of  small  strongly  refractive  particles,  because 
the  light  which  enters  it,  passing  alternately  from  a  more  strongly  refractive  particle 
into  the  less  refractive  substance  which  surrounds  it,  and  then  again  into  another  par- 
ticle, and  so  on,  finally  becomes  to  a  certain  extent  lost,  a  part  only  reaching  the  eye, 
and  that  after  a  roundabout  course  among  the  particles. 


INTRODUCTION.  1 1 

chloroform.  The  pure  commercial  balsam  is  thinned  with  chloroform 
until  it  will  drop  readily  from  the  end  of  a  glass  rod.  It  is  to  be  kept 
in  tightly  stoppered  bottles.  The  mode  of  procedure  in  Canada 
balsam  mounting  is  the  following  :  the  specimen  having  been  suit- 
ably prepared  and  stained,  it  is  freed  as  completely  as  possible  from 
water  by  touching  its  edges  with  a  bit  of  fine  blotting-paper,  and  then 
laid  into  a  dish  containing  a  few  cubic  centimetres  of  common  strong 
alcohol ;  after  from  ten  to  fifteen  minutes  it  is  transferred  to  absolute 
alcohol.  It  lies  in  this  for  from  ten  to  fifteen  minutes,  when  it  is  taken 
out,  the  superfluous  alcohol  removed  as  before,  and  laid  in  oil  of  cloves. 
As  soon  as  it  becomes  transparent,  which  will  usually  occur  in  from 
five  to  ten  minutes,  the  excess  of  oil  is  removed  with  blotting-paper 
and  the  section  is  transferred  to  a  drop  of  balsam  on  a  slide  and  cov- 
ered. This  process,  which  seems  at  first  somewhat  complicated,  is 
readily  understood  when  we  remember  that  neither  the  water  in  which 
the  specimen  usually  lies  when  the  staining  is  completed,  nor  the  alco- 
hol, which  is  very  hygroscopic  and  removes  the  water  from  the  speci- 
men, are  miseible  with  balsam ;  and,  further,  that  alcohol  is  miscible 
with  oil  of  cloves,  and  this  with  balsam.  Care  must  be  taken,  in  these 
manipulations,  not  to  breathe  on  the  specimen,  nor  to  allow  any 
moisture  to  come  in  contact  with  it,  since  even  a  small  amount  of 
moisture  causes  a  precipitate  in  the  balsam  and  renders  the  specimen 
well-nigh  worthless,  unless  it  be  remounted.  Canada  balsam  is,  as  a 
rule,  best  adapted  for  mounting  those  specimens  in  which  we  wish  to 
study  the  general  structure  of  a  tissue  rather  than  its  more  minute 
characters.  Preparations  in  which  the  blood  and  lymphatic  vessels 
have  been  injected  with  some  colored  material  usually  show  well  when 
mounted  in  balsam.  In  a  very  large  number  of  cases,  a  double  stain- 
ing of  the  specimen  with  hematoxylin  and  eosin,  before  mounting  in 
balsam,  gives  excellent  results.  To  accomplish  this,  we  may  first 
stain  in  the  usual  way  with  hematoxylin,  and  accomplish  the  eosin 
staining  by  adding  a  small  amount  of  a  solution  of  eosin  in  absolute 
alcohol  to  the  absolute  alcohol  used  for  the  dehydration  of  the  speci- 
men, before  laying  it  in  balsam. 

Acetic  acid  is  an  agent  of  great  value  in  certain  examinations  of 
tissue.  It,  too,  possesses  the  power  of  rendering  certain  tissues  trans- 
parent. In  accomplishing  this  effect,  however,  it  acts  in  an  entirely 
different  way  from  the  above  mentioned  agents.  It  does  not  simply 
surround  the  structures  with  a  strongly  refractive  substance,  but  it 
causes  albuminoid  particles  to  swell  up  and  become  actually  more 
transparent.  Moreover,  it  does  not  act  alike  upon  all  parts  of  tissues  ; 
for  example,  while  it  renders  cell-bodies  more  transparent,  it  does  not 
produce  the  same  effect  upon  the  nuclei,  but  rather  causes  them  to  be- 
come more  distinct,  through  actual  shrinkage  and  the  greater  contrast 
which  they  present  after  the  change  to  the  transparent  cell-bodies.  It 
is  usually  employed  in  the  study  of  fresh  tissues,  and  is  much  less  used 
than  formerly,  on  account  of  the  very  essential  changes  in  form  which 
it  is  now  known  to  induce  in  the  tissue-elements.  It  may  be  used  in 
solutions  of  from  two  to  three  per  cent. 

The  chemical  agents  which  the  histologist  uses,  and  the  manipula- 


12  NORMAL  HISTOLOGY. 

tive  devices  to  which  he  has  recourse  in  the  study  of  the  tissues,  are 
very  numerous,  and  we  have  considered  here  only  a  few  of  the  more 
important  and  typical.  We  shall  acquaint  ourselves  with  others  as  we 
proceed  with  the  practical  study.  The  preparation  of  each  tissue 
presents  to  the  worker  in  histology  a  separate  problem,  and  in  few  de- 
partments of  science  is  careful  attention  to  technical  minutiae  of  more 
importance  than  in  that  which  now  engages  us. 


CHAPTER   I. 

THE   CELL   IN   GENERAL. 

In  all  animal  bodies  are  found  certain  tiny  structural  elements, 
called  cells ;  and  before  commencing  the  systematic  study  of  the  tis- 
sues, it  is  necessary  to  obtain  some  definite  conception  of  the  nature 
of  these  elementary  organisms.  We  may  consider  cells  from  a  mor- 
phological and  from  a  physiological  standpoint.  What  is  the  structure 
of  cells,  and  what  do  they  do  ?  First,  then,  what  is  the  structure  of 
cells  ?  We  find  a  great  diversity  in  the  structure  of  animal  cells,  some 
being  extremely  simple,  others  quite  complex.  The  most  common, 
and  usually  the  most  prominent  structural  feature  of  the  cell,  that 
portion  which  gives  to  it  form  and  consistence,  is  called  the  cell-body. 
This  usually  consists  of  an  albuminoid  material,  sometimes  transpar- 
ent and  apparently  structureless,  sometimes  finely  or  coarsely  granu- 
lar, and  not  infrequently  presenting,  at  least  after  death,  a  reticulated 
appearance;  this  material  is  called  protoplasm  in  its  typical  active 
form,  and  may  be  then  very  soft  and  viscid,  like  thin  jelly  ;  or  it  may 
become  so  modified  as  to  be  hard,  and  even  horny  like  the  nail.  The 
cell-body  presents  a  great  variety  of  forms  ;  it  may  be  spherical,  cu- 
boidal,  cylindrical,  fusiform,  ovoid,  pear-shaped,  discoidal,  or  scale- 
like ;  it  often  sends  off  processes  like  branches  or  wings,  and  some- 
times assumes  the  most  irregular  bizarre  forms.  We  not  infrequently 
find  embedded  in  the  cell-body,  pigment-granules,  droplets  of  fat,  and 
various  kinds  of  crystals.  The  form  of  the  cell-body  seems  usually  to 
depend  largely  upon  the  pressure  to  which  it  is  or  has  been  subjected 
by  adjacent  structures. 

Within  the  cell-body  we  usually  find,  centrally  placed  or  at  the 
side,  one  or  more  spherical,  ovoidal,  or  irregular-shaped  bodies,  called 
nuclei  (singular,  nucleus).  The  nucleus  usually  presents  a  sharply  de- 
fined contour,  is  frequently  coarsely  granular,  and  sometimes  a  coarse 
or  filiform  network  is  seen  suspended  within  or  stretching  from  side  to 
side.  The  nucleus  may  be  very  small  in  proportion  to  the  size  of  the 
cell-body,  or  may  make  up  nearly  the  entire  bulk  of  the  cell.  In  the 
processes  of  degeneration  and  decomposition,  and  under  the  action  of 
certain  chemical  agents,  the  nucleus  seems  to  be  more  resistant  than 
the  cell-body,  and  on  treatment  of  the  cell  with  certain  coloring 
agents,  such  as  carmine  and  hematoxylin,  the  nucleus  is  more  deeply 
stained  than  the  cell-body.  Within  the  nucleus,  again,  we  frequently 
find  one  or  more  small  spherical  or  irregular-shaped  bodies,  looking 
like   vesicles  or   shining  granules,  which    are  called  nucleoli.     They 


14  NORMAL   HISTOLOGY. 

would  seem,  in  some  cases  at  least,  to  be  connected  with  the  above- 
mentioned  intra-nuclear  network.  Of  the  exact  nature  and  signifi- 
cance of  the  nucleus  and  nucleolus,  we  have,  at  present,  little  definite 
knowledge.  Finally,  a  small  proportion  of  animal  cells  are  enclosed 
by  an  envelope — the  cell-membrane,  which  may  be  thick  or  thin,  now 
presenting  well-defined  structural  peculiarities,  and  again  quite  homo- 
geneous and  structureless.  In  many  cases  the  cell-membrane  would 
seem  to  be  simply  a  peripheral  hardened  layer  of  the  cell-body.  It 
will  thus  be  seen  that  we  may  have,  in  an  animal  cell,  four  distinct 
structural  elements  :  the  body,  the  nucleus,  the  nucleolus,  and  the  mem- 
brane. It  is  only  in  a  few  varieties  of  cells,  however,  that  all  of  these 
elements  are  present.  The  cell-membrane  is  the  least  commonly  pre- 
sent of  all,  and  there  are  certain  cells  which  consist  of  a  cell-body  alone. 

Regarding  cells  now  from  a  physiological  point  of  view,  we  find 
the  expression  of  their  vitality  in  four  distinct  ways  :  they  nourish 
themselves  ;  they  grow  ;  they  perform  certain  functions  for  their  own 
good  and  for  the  benefit  of  the  organism  of  which  they  form  a  part ; 
and  finally,  under  certain  circumstances,  they  are  capable  of  repro- 
ducing their  like.  Or,  in  more  concise  language,  we  say  :  the  cell  ex- 
presses its  vitality  in  nutrition,  growth,  function,  and  reproduction. 
Not  all  of  these  expressions  of  vitality,  however,  can  be  subjected  to 
direct  microscopical  observation.  Nutrition  being,  as  we  believe,  es- 
sentially a  chemical  process,  cannot  become  the  subject  of  direct 
microscopical  study,  at  least  until  very  essential,  and  at  present  ap- 
parently impossible,  improvements  shall  have  been  made  in  our 
technique.  The  growth  of  cells  too,  is  for  the  most  part  so  gradual, 
that  we  cannot  directly  follow  it,  but  are  obliged  to  study  it  in  differ- 
ent phases  of  its  progress. 

The  functional  activity  of  cells  can  be  indirectly  subjected  to  mi- 
croscopical investigation  when  it  is  associated  with  demonstrable 
changes  in  the  morphological  characters  of  the  cell,  or  directly  ob- 
served when  it  expresses  itself  in  motion ;  thus,  we  can  readily  detect 
the  difference  between  a  condition  of  functional  activity  and  rest  in 
the  peptic  cells  of  the  stomach ;  and  the  observation  of  ciliary  and 
amoeboid  movements  in  certain  cells  are  among  the  most  fascinating 
of  histological  studies.  Finally,  regarding  the  reproduction  of  cells, 
in  a  few  instances  the  act  has  been  directly  observed  under  the  micro- 
scope, but  in  the  majority  of  cases  our  knowledge  is  derived  from  the 
study  of  a  succession  of  consecutive  stages  in  the  process.  Every 
new  cell  which  appears  in  the  animal  body,  during  and  subsequently 
to  its  development,  is  derived  from  a  pre-existing  cell,  and  all  the  cells 
are  derivatives  of  a  single  original  cell,  the  ovum.  New  cells  are  pro- 
duced by  a  process  of  division  in  older  cells.  Cell-division  seems  to 
occur  in  a  variety  of  ways,  usually  commencing  in  the  nucleus.  Of 
the  details  of  the  process  we  have  at  present  but  little  satisfactory 
classified  knowledge,  and  although  interesting  and  valuable  facts  are 
rapidly  accumulating,  the  scope  of  these  lessons  will  not  permit  us  to 
tarry  long  upon  the  subject. 

In  one  of  the  apparently  simplest  modes  of  cell-division,  one  or 
more  constrictions  appear  around  the  cell-body,  either  before  or  after 


THE   CELL   IN   GENERAL.  1 5 

certain  changes  in  the  nucleus  have  led  to  its  partial  or  entire  division  ; 
these  constrictions  grow  deeper  and  deeper,  until  finally  the  parts  sepa- 
rate and  become  individual  cells.  This  variety  of  cell-divison  is  fre- 
quently termed  reproduction  by  fission. 

If  division  occurs  in  cells  which  are  enclosed  in  a  distinct  mem- 
brane or  envelope,  so  that  the  new  organisms  are  not  at  once  set  free, 
the  process  is  called  endogenous  cell-reproduction. 

Or  finally,  as  frequently  occurs,  the  cell-body  may  send  off  from 
some  part  a  bud-like  process,  which  by  a  constriction  of  its  pedicle  is 
at  length  freed  ;  this  process  is  called  cell-reproduction  by  budding 
or  gemmation. 

These  modes  of  cell-reproduction  seem  to  be  different,  and  yet 
there  is  much  reason  for  believing  that  they  are  really  only  modifica- 
tions of  one  process ;  but  in  the  present  condition  of  our  knowledge 
on  the  subject  all  general  statements  should  be  very  cautiously  made, 
and  such  classifications  as  the  above  are  to  be  regarded  merely  as  for 
convenience  of  study. 

Cells  are  variously  classified  according  to  their  nature  and  relation 
to  adjacent  parts  :  thus,  we  have  epithelial  cells,  which  cover  the  skin 
and  mucous  membranes  and  occur  in  certain  parts  of  the  glandular 
organs;  connective-tissue  cells,  which  lie  scattered  through  the  sub- 
stance of  the  structures  presently  to  be  studied  as  connective  tissue, 
in  certain  parts  undergoing  modification  of  form  and  relation  to  neigh- 
boring parts  and  called  endothelial  cells  ;  gland-cells  are  those  which, 
possessing  peculiar  functional  or  morphological  characters,  make  up 
the  tissue  of  the  parenchyma  of  certain  glands  and  organs.  There  are 
other  classes  of  cells  which  will  be  considered  in  our  systematic  study 
of  the  tissues. 

PRACTICAL   STUDY. 

Many  of  the  general  characters  of  cells  may  be  learned  from  the  study 
of  the  epithelial  cells  of  the  bladder,  the  pigmented  cells  of  the  retina, 
and  the  ciliated  cells  from  the  mucous  membrane  of  the  frog's  mouth. 

Epithelial  Cells  from  the  Rabbit's  Bladder. — The  bladder  of  a  re- 
cently killed  rabbit  is  opened  and  the  mucous  membrane  scraped  with  a 
scalpel.  The  grayish  mass  which  is  scraped  off  consists  almost  entirely 
of  cells.  It  is  put  immediately  into  a  mixture  of  alcohol  (85  per  cent.), 
one  part,  water  two  parts,  and  allowed  to  remain  for  twenty-four 
hours.  A  tiny  bit  of  the  cell-mass  is  then  placed  on  a  slide  with  a 
drop  of  a  dilute  aqueous  solution  of  eosin,  and  teased  into  extremely 
fine  fragments  with  needles.  By  the  time  the  cells  are  sufficiently 
separated,  they  will  have  become  stained  of  a  light  rose-red  color. 

A  small  drop  of  glycerine  is  now  mixed  with  the  fluid  on  the  slide, 
and  the  whole  carefully  covered  with  a  thin  glass  so  as  to  exclude 
all  bubbles  of  air. 

The  specimen  is  now  ready  for  study,  and  should  be  examined  first 
with  a  low  and  then  with  a  high  power.  The  cells  will  be  found  to 
have  a  great  variety  of  forms,  some  of  them  evidently  determined  by 
the  pressure  of  adjacent  cells.  They  possess  a  finely  granular  cell- 
body,  with  more  coarsely  granular  nuclei,  and  nucleoli.    If  it  be  desired 


1 6  NORMAL  HISTOLOGY. 

to  preserve  the  specimen,  a  narrow  rim  of  asphalt  varnish  is  painted 
around  the  cover-glass,  covering  the  edges  of  the  latter  and  extending 
for  a  short  distance  on  to  the  slide. 

Pigmented  Cells  of  the  Retina. — A  fresh  eye  (from  the  ox  or  sheep), 
or  better,  one  which  has  lain  for  a  few  days  in  Miiller's  fluid,  is  opened 
by  an  equatorial  section,  and  the  vitreous  body  and  the  retina  removed 
from  the  posterior  half.  If  the  remaining  portion,  including  the  sclera, 
choroid,  and  a  portion  of  the  pigmented  epithelium  of  the  retina,  be 
put  under  water  in  a  shallow  dish,  and  brushed  gently  over  the  surface 
with  a  fine  camel's-hair  pencil,  delicate  pigmented  flakes  will  float  off 
in  the  water.  These  are  the  desired  cells.  One  or  two  bits  should 
be  put  into  a  drop  of  glycerine  on  a  slide.  Another  bit  is  put  on  to 
another  slide  with  a  drop  of  haematoxylon  solution,  and  after  about  ten 
minutes  the  coloring  fluid  carefully  washed  off  with  water,  and  the 
stained  fragments  placed  with  the  other  in  the  glycerine  on  the  other 
slide.  They  are  now  covered  and  examined.  These  cells  are  flat, 
hexagonal,  and  joined  together  edge  to  edge,  giving  a  pavemented 
appearance  to  the  fragments. 

Most  of  the  cell-bodies  are  closely  crowded  with  irregular-shaped 
brown  or  black  pigment-granules.  Sometimes  such  are  seen  as  have 
but  few  granules  within  them,  and  occasionally  they  are  entirely  free. 
When  the  cell-bodies  are  crowded  with  pigment,  the  nucleus  appears 
as  an  irregular,  sharply  outlined  structure  containing  no  pigment  and 
looking  like  a  hole  in  the  cell.  When  the  pigment  is  not  present  in 
considerable  quantity,  or  not  at  all,  the  nucleus  is  much  less  clearly 
outlined.  In  cells  which  have  been  stained  with  hematoxylin,  how- 
ever, the  nuclei  all  present  well  defined  outlines  and  are  stained  of  a 
violet  color. 

Ciliated  Cells  from  the  Frog's  Month. — The  mucous  membrane  of 
the  roof  of  the  mouth  or  the  gullet  of  a  living  or  recently  killed  frog 
is  gently  scraped  with  a  scalpel.  The  slimy  mass  which  is  thus  pro- 
cured is  transferred  to  a  drop  of  one-half  per  cent,  salt  solution,  on  a 
slide,  and  thoroughly  teased  apart  with  needles.  The  specimen  is 
now  covered,  a  bit  of  hair  being  placed  beforehand  beside  the  speci- 
men to  prevent  the  cover-glass  from  pressing  on  the  cells.  The  cells 
are  spheroidal  or  shortly  columnar,  and  will  be  seen  isolated  or  in 
groups,  the  cilia  springing,  in  the  form  of  delicate  rows  of  hair-like 
processes,  from  the  side  of  the  cell  which,  when  the  latter  is  in  situ, 
lies  at  the  free  surface  of  the  mucous  membrane.  A  considerable 
number  of  non-ciliated  cells  will  also  be  observed.  The  form  of  the 
cilia  will  be  most  readily  seen  in  cells  in  which  the  movement  is 
becoming  languid,  which  occurs  gradually  in  all  the  cells,  and  finally 
ceases  altogether.  The  movement,  when  vigorous,  often  causes  cells 
and  masses  of  cells  to  revolve  and  move  about  in  the  fluid  and  fre- 
quently causes  currents  in  the  latter,  into  which  floating  particles  are 
drawn  and  then  driven  onward  with  considerable  velocity.  If  it  be 
desired  to  preserve  specimens  of  the  ciliated  cells,  portions  of  the 
mucous  membrane  may  be  scraped  as  before,  and  the  mass  treated 
with  dilute  alcohol  and  stained  and  mounted  as  above  directed  for  the 
epithelial  cells  of  the  bladder. 


CHAPTER  II. 

CONNECTIVE    TISSUE. 

a.  Intercellular  Substance. 

Our  knowledge  of  the  animal  tissues  is  not  yet  extensive  and  exact 
enough  to  enable  us  to  make  a  satisfactory  classification  of  them,  but 
for  convenience  of  study  we  may  regard  the  body  as  composed  of 
simple  tissues  and  of  organs.  As  examples  of  the  first  we  have  con- 
nective tissue,  muscular  tissue,  nerve-tissue,  etc.  ;  of  the  second,  the 
liver,  the  lungs,  the  skin,  mucous  membranes,  etc.  Among  the  simple 
tissues  there  is  a  large  and  important  group,  called  connective  tissues, 
the  members  of  which,  though  presenting  many  marked  differences, 
yet  seem  so  closely  allied,  both  in  structure  and  life  history,  as  to 
justify  their  grouping  under  the  above  common  name.  The  members 
of  this  group  of  tissues  may  be  tabulated  as  follows  :  * 
i.   Fibrillar  connective  tissue. 

2.  Embryonal  and  mucous  tissue. 

3.  Fat  tissue. 

4.  Reticular  connective  tissue. 

5.  Cartilage. 

6.  Bone  and  teeth. 

Aside  from  many  striking  points  of  similarity  in  their  structure, 
which  we  can  better  appreciate  after  having  made  a  practical  study  of 
the  group,  three  considerations  may  be  briefly  noticed  here  as  of 
weight  in  determining  this  classification.  At  an  early  period  of  em- 
bryonic life,  when  the  animal  is  composed  almost  exclusively  of  cells, 
it  is  found  that  one  of  the  first  definite  arrangements  or  groupings  of 
these  cells  consists  in  the  formation  of  three  distinct  layers:  an  outer, 
a  middle,  and  an  inner  layer.  Now  it  is  found  that  the  cells  in  these 
different  layers  have  an  entirely  distinct  capacity  in  their  power  of 
producing  the  tissues  which  are  found  in  the  adult  animal.  From  the 
cells  in  the  outer  layer  are  produced  the  skin  with  its  adnexa,  certain 
organs  of  sense,  and  the  central  nervous  system  ;  from  the  inner  layer 
are  formed  the  intestinal  epithelium  and  glands,  and  certain  of  the 
large  organs  of  the  body ;  while  from  the  middle  layer  are  developed 

*  In  addition  to  these  varieties  we  find  in  certain  parts  of  the  body  membranous 
layers  or  sheaths — sometimes  structureless,  sometimes  having  well-marked  structural 
features — which  differ  in  many  respects  from  the  above  varieties  of  connective  tissue, 
but  whose  separate  consideration  here  would  carry  us  beyond  the  scope  which  time 
permits  these  lessons  to  assume.  They  will  be  briefly  considered  as  we  meet  with 
them  in  our  systematic  study  of  the  parts  of  the  body  in  which  they  occur. 
2 


1 8  NORMAL  HISTOLOGY. 

the  muscles,  the  vascular  systems,  and,  which  especially  concerns  us 
here,  the  tissues  tabulated  above  as  members  of  the  connective-tissue 
group. 

Besides  the  relationship  given  to  them  by  this  common  origin, 
these  tissues  show  their  close  alliance  by  the  fact  that  during  the  pro- 
cess of  development  one  is  sometimes  formed  from  another.  Finally, 
certain  frequently  observed  pathological  conditions  seem  to  consist 
chiefly  in  the  transformation  of  one  of  these  forms  of  tissue  into 
another  of  the  same  group. 

With  fibrillar  connective  tissue  or  connective  tissue  proper,  or  sim- 
ply connective  tissue,  as  it  is  often  called,  we  commence  our  systematic 
study.  This  tissue  is  most  widely  distributed  in  the  human  body, 
occurring  in  the  greatest  diversity  of  forms,  and  an  exact  knowledge 
of  its  structure  and  arrangement  is  absolutely  essential  to  a  correct 
understanding  of  most  other  tissues  and  the  organs,  since  it  occurs  in 
one  form  or  another  in  nearly  all  of  them.  Fibrillar  connective  tissue, 
under  the  name  of  tendons  and  ligaments,  forms  bands  and  cords 
which  bind  the  muscles  to  the  bones,  and  bind  the  bones  together.  It 
is  spread  out  in  thin  layers  in  the  fasciae  and  aponeuroses  ;  it  surrounds 
the  bones  and  cartilage  as  periosteum  and  perichondrium.  It  divides 
and  encloses  muscular  bundles  and  the  nerves ;  it  supports  the  blood 
and  lymphatic  vessels,  and  forms  the  limiting  membrane  of  the  serous 
cavities.  It  forms  an  encasing  membrane  for  many  organs,  and,  ex- 
tending into  their  interior,  serves,  under  the  name  of  interstitial  tissue, 
to  support  their  parenchyma.  Everywhere  it  performs  these  chief  and 
essential  functions;  it  supports,  it  binds  parts  _  together,  it  protects. 
Fibrillar  connective  tissue  is  composed  of  two  distinct  classes  of  struc- 
tural elements  :  a.  cells,  and  b.  a  substance  lying  between  the  cells, 
the  intercellular  or  basement  substance.  The  intercellular  substance 
consists  chiefly  of  two  distinct  kinds  of  fibres  :  fibrillated  fibres  and 
elastic  fibres.  The  fibrillated  fibres  are  tiny,  grayish,  translucent  cords, 
which,  sometimes  singly,  sometimes  in  straight  or  wavy  bundles,  lie 
nearly  parallel  to  one  another,  and  again  cross  each  other  at  all  con- 
ceivable angles,  forming  complicated  net-works.  When  examined 
fresh,  with  high  magnifying  powers,  the  individual  fibres  are  seen  to  be 
moderately  refractive  ;  they  have  a  delicate  longitudinal  striation — this 
striation  being,  as  we  shall  presently  see,  the  expression  of  the  fact 
that  each  fibre  is  made  up  of  a  number  of  still  finer  fibrillar.  These 
fibrillated  fibres,  although  frequently  crossing  one  another,  and  intri- 
cately interlacing,  do  not,  as  a  rule,  branch  nor  form  anastomoses  with 
one  another.  On  boiling  for  a  considerable  time  in  water  they  are 
converted  into  gelatine,  and  when  treated  with  acetic  acid  or  dilute 
alkalies  they  swell  up,  lose  their  longitudinal  striations,  become  very 
transparent,  and  finally  almost  invisible. 

It  is  not  easy,  in  studying  fresh  tissues,  to  convince  one's  self 
that  the  longitudinal  striations  on  the  fibres  are  really  the  expression 
of  their  fibrillar  structure,  since  any  attempt  to  pick  the  fibres  apart 
with  needles  is  of  little  avail,  because  the  fibrillar  are  bound  together 
by  3  minimal  amount  of  cement  substance. 

Li,  however,  the  tissue  be  placed  for  a  few  hours  in  some  fluid 


CONNECTIVE    TISSUE.  19 

which  dissolves  this  cement  substance,  as  picric  or  osmic  acid,  the 
ultimate  elements  may  be  readily  separated  by  teasing.  The  second 
variety  of  fibres  which  occur  in  connective  tissue — the  elastic  fibres — 
are  much  more  strongly  refractive  than  the  first,  hence  presenting 
more  sharply  marked  contours  ;  they  are  not  longitudinally  striated, 
and  are  not  usually  grouped  together  in  bundles ;  they  often  branch 
and  form  anastomoses  with  one  another,  sometimes  joining  at  frequent 
intervals  to  form  a  narrow-meshed  net,  and  again  stretching  away  for 
long  distances  to  form  a  broadly-spaced  reticulum.  Sometimes  the 
fibres  are  broad  and  band-like,  sometimes  extremely  fine  ;  on  being 
boiled  in  water  they  are  not  converted  into  gelatine,  and  they  are  un- 
changed by  acetic  and  dilute  alkalies.  These  fibres,  as  their  name 
indicates,  possess  elasticity,  and  as  a  consequence  of  this  property  we 
often  find,  when  the  fibres  have  been  severed  by  teasing  or  other 
modes  of  preparation,  that  the  free  ends  curl  over  in  the  act  of  retrac- 
tion, forming  very  characteristic  curves  or  spirals.  The  elastic  sub- 
stance sometimes  occurs  in  the  form  of  granules  instead  of  fibres. 
The  relative  number  of  the  fibrillar  and  elastic  fibres  varies  greatly  in 
different  tissues  :  in  some  we  find  but  few  elastic  fibres,  others  contain 
little  else.  The  interstices  of  the  interlacing  fibres  are  filled  with  the 
nutritive  fluids  of  the  body,  or  in  some  cases  a  small  amount  of  a 
more  consistent  homogeneous  material  binds  them  together.  The 
marked  difference  in  general  appearance,  which  is  seen  in  different 
parts  composed  of  connective  tissue,  is  due  to  differences  in  the 
arrangement  of  the  fibres  and  bundles  and  the  relative  proportion  of 
the  fibrillated  and  elastic  fibres. 

PRACTICAL    STUDY. 

Subcutaneous  connective  tissue. — To  study  the  basement  substance 
of  fibrillar  connective  tissue,  the  skin  should  be  reflected  back  from 
the  abdominal  wall  of  a  recently  killed  rabbit  or  dog,  and,  choosing  a 
part  which  is  free  from  fat,  a  bit  of  the  loose  subcutaneous  tissue  is 
seized  with  the  forceps  and  snipped  off  with  scissors.  The  bit  of  tis- 
sue, which  will -contract  to  a  little  lump  around  the  point  of  the  forceps, 
is  now  to  be  carefully  spread  out  on  a  slide  and  covered  with  one-half 
per  cent,  salt  solution. 

The  specimen  will  be  seen  to  contain  a  large  proportion  of  fibril- 
lated fibres  crossing  one  another  in  all  directions,  with  a  few  narrow 
elastic  fibres.  After  studying  in  salt  solution,  a  drop  of  two  per  cent, 
solution  of  acetic  acid  should  be  allowed  to  run  under  one  edge  of  the 
cover-glass,  the  salt  solution  being  drawn  off  by  a  bit  of  filter  paper 
placed  at  the  opposite  edge,  and  the  effect  carefully  observed. 

Tail  Tendon  of  Mouse. — In  order  to  demonstrate  that  the  striated 
fibres  are  composed  of  finer  fibrillae,  a  bit  of  the  tail  tendon  of  a  mouse 
or  a  small  tendon  from  any  animal,  should  be  soaked  for  a  few  days 
in  a  one  per  cent,  solution  of  osmic  acid,  washed  and  carefully  teased 
apart  on  a  slide  in  glycerine.  To  make  a  permanent  preparation  the 
cover  may  be  surrounded  by  a  rim  of  asphalt  varnish. 

Adventitia  of  the  Aorta. — A  shred  should  be  stripped  from  the 


20  NORMAL   HISTOLOGY. 

outer  layers  of  the  aorta  of  any  mammal,  either  fresh  or  after  it  has 
been  in  preservative  fluids,  carefully  teased  on  a  slide  and  mounted  in 
glycerine.  In  this  specimen  the  elastic  fibres  preponderate  in  the 
basement  substance. 

Connective  Tissue. 
b.  Cells. 

We  consider  next  the  cellular  elements  of  fibrillar  connective 
tissue.  Two  distinct  classes  of  cells  are  found  here.  First  :  those 
which  are  essential  components  of  it,  preserving  a  fixed  and  definite 
relation  to  the  basement  substance,  and  being  tolerably  constant  in 
the  different  varieties  of  tissue,  in  form,  size,  and  number,  the  so- 
called  fixed  connective-tissue  cells;  and  second,  small  spherical  cells, 
resembling  in  all  their  characters  the  white  blood-cells,  with  which  they 
are  believed  to  be  identical.  These  latter  cells  escape  from  the  blood- 
vessels and  wander  through  the  tissues,  in  which  they  are  found  in 
varying  number  under  normal  conditions,  and  are  called  wandering 
cells.  These  cells  will  be  studied  in  connection  with  the  blood. 
Among  the  fixed  connective-tissue  cells,  by  far  the  greater  proportion 
are  more  or  less  flattened,  presenting  to  the  eye,  when  seen  from  the 
edge  or  in  cross-section,  the  appearance  of  slender  spindles.  The 
cell-bodies  are  for  the  most  part  quite  transparent,  often  very  thin 
and  scale-like,  frequently  so  delicate  as  to  be  difficult  of  recogni- 
tion, and  sometimes  the  protoplasm  in  the  vicinity  of  the  nucleus  is 
distinctly  granular.  These  fixed  connective-tissue  cells  present  a  great 
variety  of  forms,  being  round,  ovoid,  oblong,  fusiform  or  irregularly 
rectangular,  often  sending  off  branches  which  seem  to  be  connected 
with  the  branches  of  neighboring  cells.  Sometimes  the  thin  cell-body 
sends  off  one  or  more  delicate  wing-like  processes  at  varying  angles. 
These  cells  may  have  one  or  more  nuclei ;  they  lie  in  the  interstices 
of  the  fibres,  which  they  often  enwrap  with  their  delicate  bodies.  The 
form  which  they  assume  in  the  different  varieties  of  connective  tissue 
would  seem  to  be  dependent  upon  the  varying  conditions  of  pressure 
to  which  they  are  subjected  by  the  adjacent  fibres.  These  flattened 
connective-tissue  cells  in  many  parts  of  the  body  may,  and  in  certain 
parts  constantly  do,  contain  pigment-granules.  In  certain  parts  of 
the  body  the  connective  tissue  presents  free  surfaces,  lining  more  or 
less  well-defined  closed  spaces  or  cavities,  or  free  surfaces  which  are 
movable  over  one  another,  as  is  seen  in  the  great  serous  cavities,  in 
the  blood-  and  lymph-channels,  tendon-sheaths,  etc.  In  these  cases 
the  flat  connective-tissue  cells  usually  undergo  some  modification  in 
their  form,  character,  and  relations  to  one  another,  and  are  called 
endothelial  cells  or  endothelium.  Finally,  in  certain  parts  of  the  body, 
usually  in  the  vicinity  of  blood-vessels,  are  found  irregular-shaped, 
granular  cells,  not  flat,  of  varying  size  and  form,  which  resemble  in 
many  respects  cells  found  in  the  embryo,  but  whose  nature  and  pur- 
pose is  little  understood.     These  are  sometimes  called  plasma  cells. 

The  various  forms  of  connective-tissue  cells  and  their  relations  to 


CONNECTIVE    TISSUE.  21 

the  intercellular  substance  may  be  studied  by  considering,  somewhat 
in  detail,  a  few  different  and  typical  forms  of  connective  tissue.  Those 
which  we  shall  employ  are  :  the  subcutaneous  tissue,  the  choroid,  tendons, 
the  cornea,  and  for  the  study  of  endothelial  cells,  the  serous  membranes 
of  the  peritoneum. 

PRACTICAL    STUDY. 

Cells  in  the  Subcutaneous  Connective  Tissue. — The  fixed  connective- 
tissue  cells  which  occur  in  that  form  of  tissue  in  which  the  basement 
substance  is  loosely  arranged  in  crossing  or  interlacing  fibres  or  bundles 
— often  called  areolar  tissue — may  be  studied  in  the  subcutaneous  tis- 
sue or  fasciae  of  such  animals  as  the  rabbit,  dog,  or  guinea-pig,  or  in 
man.  The  cells  vary  somewhat  in  size,  shape,  and  number,  in  the 
different  animals,  but  those  in  the  rabbit  may  be  regarded  as  typical. 
In  the  study  of  these  cells,  whose  bodies  are  for  the  most  part  so  thin 
and  transparent  as  to  be  almost  invisible  even  with  high  magnifying 
powers,  we  have  to  fulfil  two  important  indications  in  our  technical 
procedure;  we  must  treat  the  tissue  with  some  agent  which  will 
render  the  cells  visible,  and  at  the  same  time  the  form  of  the  delicate 
cell-body  must  remain  as  nearly  as  possible  unchanged.  These  ends 
may  be  attained  in  the  following  way  :  the  skin  being  reflected  back 
from  the  abdominal  walls  of  a  freshly  killed  rabbit,  with  an  ordinary 
hypodermic  syringe,  inject  beneath  the  loose  adherent  tissue  several 
drops  of  an  aqueous  solution  of  nitrate  of  silver,  i  part  to  1,000  ;  a 
circumscribed  cedematous  area  will  be  thus  produced,  from  which  a  bit 
may  be  snipped  off  with  scissors,  spread  out  in  a  thin  layer  on  a  slide, 
covered  with  one-half  per  cent,  salt  solution,  and  studied.  The  silver 
serves  to  fix  the  cells  in  their  natural  form  without  much  shrinkage, 
and  renders  the  cell-body  faintly  visible.  This  method  of  bringing  re- 
agents into  contact  with  the  tissues,  is  called  the  method  of  interstitial 
injection.  In  order  to  bring  the  cells  still  more  clearly  into  view,  the 
specimen  may  be  stained,  first  with  a  very  dilute  solution  of  hema- 
toxylin, and  then  with  a  dilute  aqueous  solution  of  eosin ;  the  nuclei 
will  then  be  seen  colored  violet,  the  cell-bodies  a  light  rose-red.  The 
cells  in  this  tissue  are  for  the  most  part  extremely  thin,  and  of  various, 
often  quite  irregular  forms,  sometimes  sending  off  narrow  processes  by 
which  they  join  neighboring  cells,  and  sometimes  furnished  with  wing- 
like projections.  Nerves  and  capillary  blood-vessels  are  sometimes 
seen  in  the  specimens,  and  occasionally  in  the  vicinity  of  the  latter  are 
found  the  above-mentioned  plasma  cells.  The  specimens  may  be  per- 
manently preserved  in  glycerine  and  enclosed  by  a  rim  of  asphalt 
varnish. 

Connective-tissue  Cells  of  the  Choroid. — Pigmented  connective-tis- 
sue cells  may  be  studied  in  the  choroid  of  the  eye  of  any  of  the  mam- 
malia (except  of  albinos,  where  the  choroid  cells  are  unpigmented). 
A  shred  of  the  choroid  should  be  torn  off  with  forceps,  and  one  bit 
spread  out  and  studied  in  glycerine  unstained,  another  stained  with 
hematoxylin.  Irregular-shaped,  often  somewhat  branched  and  flat- 
tened cells,  are  seen  lying  embedded  in  a  membranous  basement  sub- 


22  NORMAL   HISTOLOGY. 

stance  containing  elastic  fibres,  the  body  being  crowded,  except  in  the 
part  occupied  by  the  nucleus,  with  a  multitude  of  brown  or  black 
granules  ;  the  nucleus  looks  like  a  clear  space  in  the  unstained  speci- 
mens, and  is  colored  violet,  as  usual,  by  the  hematoxylin. 

Tendon,  teased. — The  tendons  are  composed  of  a  varying  number 
of  parallel  bundles  of  nbrillated  connective-tissue  fibres,  surrounded  by 
an  envelope  of  connective  tissue,  whose  fibres  are  irregularly  arranged, 
and  covered,  as  they  run  in  their  sheaths,  by  a  single  layer  of  flattened 
cells,  which  have  the  character  of  the  cells  presently  to  be  described 
as  endothelium.  The  complicated  forms  which  tendon-cells  present 
can  be  most  easily  understood  when  they  are  studied  in  their  natural 
relation  to  the  parallel  bundles  of  nbrillated  fibres  which  form  the 
intercellular  substance.  Owing  to  their  extreme  tenuity  and  the  con- 
sequent possibility  of  studying  them  with  but  little  manipulation,  the 
tail  tendons  of  the  mouse  or  rat  are  well  suited  for  this  purpose.  If 
the  skin  of  the  tail  of  a  recently  killed  mouse  or  rat  be  torn  across 
transversely  by  slight  twisting  or  pressure  with  the  edge  of  the  nail, 
and  gentle  traction  be  exerted  at  both  sides  of  the  rupture,  the  caudal 
vertebrae  will  readily  separate,  and  as  the  traction  continues  a  multi- 
tude of  extremely  fine  tendon-bundles  will  be  seen  stretching  across 
the  break.  When  the  tendons  by  this  natural  and  simple  dissection 
have  been  freed  for  one  or  two  centimetres  of  their  length,  they  are 
laid,  still  under  slight  tension,  on  a  bit  of  glass  of  suitable  size,  the 
ends  cemented  to  the  glass  with  a  bit  of  soft  wax  or  paraffine,  and  the 
remainder  of  the  tail  cut  off.  The  fastening  of  the  ends  is  to  prevent 
the  tendon  from  shrinking  when  immersed  in  the  preservative  fluid. 
The  bit  of  glass  with  its  tendon-bundles  is  now  immersed  for  twenty- 
four  hours  in  a  one  per  cent,  solution  of  osmic  acid,  then  washed  in 
pure  water.  One  or  two  of  the  tiny  bundles  is  now  to  be  carefully 
picked  into  smaller  fibres  with  needles,  laid  for  a  few  hours  in  picro- 
carmine,  washed  again,  still  further  teased  on  a  slide,  covered  with 
glycerine,  and  studied.  The  individual  bundles  of  nbrillated  fibres 
will  be  readily  seen,  and  arranged  in  rows  along  these  bundles,  edge 
to  edge,  with  more  or  less  irregular  contours,  the  flattened  tendon- 
cells  will  be  seen  to  lie.  Their  nuclei,  stained  red  by  the  picro-car- 
mine,  when  seen  from  the  side  appear  oval  or  rectangular,  and  are 
often  arranged  in  couples  along  the  fibres ;  when  seen  edgewise  they 
look  like  short,  thick  red  lines.  The  cell-bodies,  when  seen  sidewise, 
look  like  faintly  colored,  often  indefinitely  outlined,  very  thin  plates, 
stretching  out  from  the  nuclei  and  partially  enwrapping  the  tendon- 
fibres.  When  the  cell-body  is  placed  with  its  edge  toward  the  ob- 
server, and  in  close  apposition  with  the  fibre,  it  is  not  to  be  distin- 
guished, owing  to  its  extreme  thinness,  from  the  line  which  marks  the 
border  of  the  fibre  ;  if,  however,  it  has  been  separated,  in  part  or  en- 
tirely, from  the  fibre,  by  the  previous  manipulations,  it  will  appear  as 
a  faint  line,  at  some  part  of  which  is  seen  the  elongated  nucleus. 
Examining  the  specimen  still  further,  one  sees  in  the  side  view  of  the 
cells,  sometimes  along  a  single  cell,  sometimes  along  a  row  of  them, 
one  or  more  fine  lines  or  striae,  and  on  looking  carefully  at  the  ceils 
placed  edgewise,  he  can  convince  himself  that  thin,  narrow  wings  are 


CONNECTIVE    TISSUE.  23 

given  off  from  the  cell-body,  and  that  it  is  the  edges  of  these  wings 
which  cause  the  striae  in  the  cells  seen  on  the  flat.  In  teased  speci- 
mens of  tendon  one  occasionally  finds  irregular-shaped  branching 
cells.  We  have  then  in  tendon  parallel  bundles  of  fibrillated  fibres, 
arranged  along  and  in  the  interstices  between  which  lie  flattened 
cells,  whose  thin  bodies  partially  enwrap  certain  fibres  an$J  send  off 
between  other  and  neighboring  fibres  branches  and  thin  narrow  wings. 
The  interstices  between  the  bundles,  in  which  the  tendon-cells  lie,  are 
lymph-channels. 

Transverse  Sections  of  Tendon. — A  very  clear  view  of  the  rela- 
tions between  the  cells  and  fibres  of  tendon,  as  well  as  the  relation  of 
the  wings  to  the  cell-bodies,  may  be  obtained  by  making  a  transverse 
section  of  a  hardened  and  stained  tendon.  Any  small  tendon  from  a 
recently  killed  animal  will  answer  the  purpose.  It  should  be  freed 
from  surrounding  tissues  and  immediately  immersed  in  a  small  quan- 
tity of  a  half  per  cent,  solution  of  gold  chloride.  Here  it  remains  for 
half  an  hour,  when  it  is  removed,  washed  with  pure  water,  and  put  into 
a  few  cubic  centimetres  of  the  following  mixture,  known  as  the  redu- 
cing fluid : 

Amyl  Alcohol 1 

Formic  Acid 1 

Water 100 


After  remaining  for  twenty-four  hours  in  the  reducing  fluid,  the 
specimen  will  probably  have  assumed  a  rich  violet  color ;  if  not,  it- 
should  be  kept  for  twenty-four  hours  longer  in  a  fresh  portion  of  the 
fluid.  When  the  requisite  color  has  been  obtained  the  specimen  is" 
hardened  for  a  day  in  alcohol,  embedded  in  hardened  liver  or  wax, 
and  thin  cross-sections  made  from  it.  The  sections  are  now  stained 
lightly  with  hasmatoxylin  and  mounted  in  glycerine.  They  should  be 
kept  as  much  as  possible  from  the  light,  which  after  a  time  destroys 
gold  preparations.  This  method  of  staining  with  gold  is  known  as 
Pritchard's  method.  The  simpler  method  of  gold-staining  known  as 
Conheim's  method  differs  from  that  just  given,  in  that,  after  soaking  in 
the  gold  solution,  the  specimen  is  exposed  to  the  light  until  it  has  as- 
sumed the  violet  color.  It  is  then  mounted  and  preserved  as  before. 
This  method  is  less  certain  in  its  results  than  Pritchard's,  being  less  apt 
to  give  clear  pictures.  In  both  methods  the  staining  action  consists 
in  the  precipitation  of  the  gold  in  metallic  form,  in  a  state  of  extreme 
comminution,  in  the  tissue.  In  one  case  this  precipitation  is  effected 
by  the  acid  mixture,  in  the  other  by  the  light.  This  deposition  of 
gold  in  the  tissues  occurs  for  the  most  part  in  the  protoplasm  of  the 
cells,  bringing  them  into  sharp  contrast  with  surrounding  parts  and 
fixing  their  form.  There  is  one  source  of  error,  however,  in  the  inter- 
pretation of  gold  specimens,  which  should  never  be  forgotten.  The 
gold  is  apparently  sometimes  deposited  and  the  characteristic  color 
produced  outside  of  the  cell  protoplasm,  and  especially  in  spaces  in  the 
tissues  which  are  filled  with  albuminoid  materials,  so  that  the  utmost 
caution  is  required  in  determining  whether  a  given  area  of  gold  deposit 


24  NORMAL   HISTOLOGY. 

and  staining  really  indicates  the  presence  of  cell  protoplasm.  And,  in 
fact,  it  is  in  some  cases  impossible,  at  present,  to  decide  this  question. 

The  Cornea — Transverse  Sections. — We  shall  employ  for  our  study 
here  the  cornea  of  the  frog,  because  of  the  ease  with  which  it  can  be 
obtained,  and  because,  on  account  of  its  thinness,  it  is  well  suited  to 
the  various  manipulations  to  which  we  shall  subject  it.  A  frog's  eye 
should  be  enucleated  directly  after  death,  and  placed  entire  in  M  tiller's 
fluid,  where  it  should  remain  for  ten  days.  The  cornea  is  then  ex- 
cised just  within  the  sclero-corneal  junction  and  laid  for  a  few  hours  in 
dilute  alcohol  (alcohol  i,  water  2),  and  then  transferred  for  a  day 
to  strong  alcohol.  Two  or  three  short  radial  incisions  are  then  made 
at  the  edge,  so  that  it  will  lie  flat.  It  is  embedded  between  two  bits 
of  liver  well  hardened  in  alcohol,  and  thin  sections  made  transversely 
across  it ;  the  sections  are  stained  first  with  dilute  hematoxylin,  and 
then  with  eosin,  and  mounted  in  glycerine. 

If  the  sections  are  made  so  as  to  include  the  entire  thickness 
of  the  cornea,  both  the  anterior  and  posterior  edges  will  be  seen 
to  be  covered  with  epithelial  cells  which  rest  upon  thin  homoge- 
neous membranes.  On  the  anterior  edge  the  cells  are  several  layers 
deep  and  have  various  forms  ;  on  the  posterior  they  are  flat  and  ar- 
ranged in  a  single  row.  Between  these  two  layers  of  cells  lies  the  con- 
nective-tissue substance  proper  of  the  cornea,  which  alone  concerns 
us  here.  The  basement  substance  of  the  cornea  consists  of  fine  fibril- 
lated  fibres  closely  bound  together  by  a  small  amount  of  cementing 
substance,  and  arranged  in  irregular  layers  or  lamellae.  Between  these 
lamellae  are  seen  the  cells  of  the  cornea,  which,  in  this  view,  seem 
to  have  the  form  of  slender  elongated  spindles,  part  of  them  being 
closely  surrounded  by  the  intercellular  substance,  part  lying  in  small 
cavities. 

La?nincB  of  Cornea. — In  order  to  determine  the  exact  form  of 
the  cells,  which  in  this  mode  of  preparation  are  seen  only  from  the 
edge,  it  is  necessary  to  study  the  cornea  as  seen  from  the  side.  For 
this  purpose  a  cornea  should  be  carefully  excised  just  within  the  sclero- 
corneal  junction,  avoiding,  as  much  as  possible,  pulling  or  stretching 
the  part.  It  is  now  stained  with  gold  chloride  by  Pritchard's  method, 
as  given  above  when  speaking  of  the  tendon.  "When  the  cornea  has 
become  violet  it  should  be  carefully  washed  and  put  into  glycerine. 
The  epithelium  on  both  the  anterior  and  posterior  surfaces  is  now 
scraped  off,  and  with  a  little  care  the  part  may  be  divided  into  several 
thin  layers  by  means  of  fine  forceps.  These  layers  are  lightly  stained 
with  haematoxylin  and  mounted  in  glycerine.  In  a  specimen  thus 
prepared,  the  basement  substance  looks  homogeneous  or  delicately 
striated  ;  fine  nerve-fibres  are  seen  stretching  across  the  specimen  in 
various  directions.  The  corneal  cells,  which  are  the  most  prominent 
objects  in  the  specimen,  are  seen  thickly  scattered  over  the  field, 
stained  of  a  reddish  violet  color.  They  have  flat,  irregular-shaped 
bodies  which  send  off  a  variable  number  of  longer  and  shorter  pro- 
cesses ;  the  nuclei  are  large,  ovoidal,  or  irregular-shaped,  and  usually 
contain  nucleoli.  Fine  irregular-branching,  almost  linear  structures 
are  seen  in  good  preparations  thickly  scattered  over  the  specimen, 


CONNECTIVE    TISSUE.  25 

and  very  frequently  lying  nearly  at  right  angles  to  one  another.  A 
part  at  least  of  these  are  continuous  with  the  cell-bodies  whose 
processes  they  are ;  whether  or  not  they  are  all  cell  processes  is  not 
yet  definitely  known.  It  will  be  seen  from  the  above  studies,  that 
the  connective-tissue  cells  of  the  cornea  are  flattened  and  branched 
cells,  and  that  certain  of  them,  at  least,  lie  in  spaces  between  the  fibres 
of  the  intercellular  substance.  We  have  now  to  study  more  carefully 
the  nature  of  these  spaces  and  their  relation  to  the  cells. 

Cornea  treated  with  Silver. — Strong  solutions  of  nitrate  of  silver 
have  the  power  of  staining  the  intercellular  substance  of  connective 
tissue  light  brown,  while  the  cells  are  left  uncolored.  In  order  to 
stain  the  cornea  of  the  frog  with  silver,  the  following  process  should 
be  employed  :  the  spinal  cord  of  the  animal  having  been  broken  up 
with  a  needle,  a  finger  is  introduced  into  the  mouth  so  as  to  press  the 
eyeball  forward  and  bring  the  cornea  into  prominence  ;  the  membrana 
nictitans  is  drawn  away  with  the  thumb  or  a  pair  of  fine  forceps,  so  as 
to  leave  the  anterior  surface  of  the  cornea  quite  free.  The  eye  is 
now  held  for  an  instant  over  a  jet  of  steam,  when  the  epithelium  will 
become  white  or  milky  in  appearance,  and  can  be  readily  scraped  off 
by  passing  the  blade  of  a  scalpel  lightly  over  the  surface.  It  is  neces- 
sary to  remove  the  epithelium  in  order  that  the  silver  solution  may 
have  ready  access  to  the  connective-tissue  substance  of  the  cornea  ; 
and  the  advantage  of  steaming  is,  that  it  can  be  scraped  off  without 
the  use  of  much  force,  which  would  disturb  the  relations  of  the  parts 
beneath.  The  epithelium  being  thus  removed,  a  five-per-cent.  solu- 
tion of  silver  nitrate  is  allowed  to  flow  over  the  cornea  and  remain  for 
two  or  three  minutes  in  contact  with  it.  By  this  treatment  the  whole 
cornea  becomes  opaque  and  stiff.  The  silver  is  now  neutralized  and 
its  further  action  prevented  by  washing  the  cornea  with  a  one-per-cent. 
salt  solution.  It  is  now  carefully  excised  and  placed  in  a  dish  con- 
taining a  mixture  of  alcohol  and  water,  1  to  2,  and  exposed  to  di- 
rect sunlight  or  bright  daylight  for  from  a. few  minutes  to  half  an  hour, 
depending  upon  the  intensity  of  the  light.  When  it  has  become 
brown  it  is  to  be  laid  in  glycerine  and  stripped  into  thin  layers,  as 
directed  above,  for  the  gold  cornea ;  the  layers  are  mounted  in  gly- 
cerine. If  the  preparation  is  successful,  clear,  branching,  communi- 
cating spaces  are  seen  on  a  yellowish  or  brown  ground.  These 
spaces,  although  larger,  evidently  correspond  in  position  to  the  cells 
in  the  cornea,  as  seen  after  treatment  with  chloride  of  gold  ;  and  if  a 
specimen  thus  prepared  be  stained  with  haematoxylin,  the  corneal  cells 
will  be  seen  lying  within  them. 

It  is  difficult  to  determine  with  certainty  whether  or  not  the  cells 
send  processes  into  all  of  the  channels  which  radiate  from  the  spaces 
in  which  they  lie;  it  is  probable  that  some  of  the  narrow  branching 
bodies  or  lines  noticed  above  in  the  gold  cornea  are  nothing  more 
than  these  branching  and  communicating  spaces  in  which  an  albu- 
minous fluid  accumulates,  which  is  stained  by  the  gold  very  much  as 
cell  protoplasm  itself  is.  We  thus  see  that  the  cornea  is  perme- 
ated by  numerous  branching  and  intercommunicating  spaces,  and 
that  in  these  spaces  the  flat,  branching  connective-tissue  cells  of  the 


26  NORMAL   HISTOLOGY. 

cornea  lie.  These  spaces  are  called  lymph-spaces,  and  what  we  have 
been  able  to  demonstrate  in  regard  to  the  relation  of  cells  to  the 
lymph-spaces  in  the  cornea  by  these  various  modes  of  preparation 
seems  to  be  true,  with  certain  modifications,  of  most  of  the  varieties 
of  connective  tissue.  These  cells  lie  in  spaces,  sometimes  com- 
pletely, sometimes  only  partially  filling  them.  These  spaces  com- 
municate with  one  another,  and  communicate  also,  on  the  one  hand, 
with  the  blood-vessels,  and,  on  the  other,  with  the  lymphatics. 
Through  them  lymph-currents  pass,  bathing  the  cells  and  supplying 
them  with  nutritive  material.  The  precise  way  in  which  these  lymph- 
spaces  (sometimes  called  serous  canaliculi)  communicate  with  the 
lymph-  and  blood-vessels  is  as  yet  unknown  to  us.  It  is  extremely 
probable  that  it  is  through  these  lymph-spaces  exclusively  that  the 
white  blood-cells  travel  in  their  peregrinations  through  the  tissues. 
We  study  these  lymph-spaces  in  the  cornea  alone,  because  the  scope 
of  these  lessons  is  too  limited  to  admit  of  such  a  detailed  study  of  all 
the  varieties  of  connective  tissue,  and  because,  while  quite  typical  of 
other  tissues,  the  relation  of  the  cells  to  the  spaces  is  here  more  clearly 
defined. 

Serous  Membranes — Endothelium. — The  so-called  serous  mem- 
branes are  formed  by  layers  of  fibrillar  connective-tissue  fibres 
mingled  with  a  varying  number  of  elastic  fibres,  and  containing  ordi- 
nary flattened  connective-tissue  cells.  They  are  more  or  less  abun- 
dantly furnished  with  blood  and  lymphatic  vessels.  On  the  free  sur- 
face of  these  membranes  rests  a  continuous  layer  of  flattened  cells, 
differing  in  many  respects  from  the  ordinary  connective-tissue  cells, 
and  called  endothelial  cells  or  endothelium.  These  cells  are  usually 
transparent,  irregularly  polygonal  in  form,  and  frequently  much  elon- 
gated ;  they  possess  one  or  more  ovoidal  nuclei,  which  often  project 
above  the  level  of  the  free  surface  of  the  cell-body  ;  they  are  placed 
edge  to  edge,  like  the  stones  in  a  mosaic,  and  seem  to  be  joined  to- 
gether by  a  minimal  amount  of  an  albuminoid  cement  substance. 

Isolated  Endothelium. — Before  studying  the  endothelial  cells  in 
situ  it  is  advisable  to  see  the  isolated  cells.  This  may  be  accom- 
plished by  macerating  any  serous  membrane,  such  as  the  pericardium 
or  peritoneum  of  a  mammal  or  the  frog,  for  a  day  or  two  in  a  mixture 
of  one-third  alcohol  to  two-thirds  water,  and  then  gently  scraping  the 
free  surface  with  a  scalpel.  The  cells  will  readily  separate  from  the 
underlying  tissue,  and  may  be  stained  with  hematoxylin  and  mounted 
in  glycerine. 

Endothelium  in  situ — Mesentery. — The  endothelial  cells,  in  their 
relation  to  adjacent  parts,  may  be  studied  in  the  mesentery  of  the 
dog,  or  rabbit,  or  frog,  and  in  the  omentum  of  the  dog. 

In  order  to  bring  the  outlines  of  the  cells  clearly  into  view,  the 
membrane  should  be  first  treated  with  a  solution  of  nitrate  of  silver. 
This  substance  in  dilute  solutions  forms  with  the  cement  substance 
between  the  endothelium  an  albuminate  of  silver,  which,  on  exposure 
to  light,  becomes  brown  or  black,  thus  clearly  defining  the  outline  of 
the  cells. 

The  mode  of  proceeding  is  as  follows  :  A  portion  of  the  mesen- 


CONNECTIVE    TISSUE.  27 

tery  of  the  recently  killed  dog  or  rabbit  should  be  carefully  removed 
and  laid  over  the  rim  of  a  shallow  dish  so  that  it  rests  loosely  over  the 
opening.  All  stretching  and  pulling  of  the  membrane  should  be 
avoided,  because  this  would  destroy  the  natural  relations  of  the  cells 
to  one  another.  The  membrane  must  be  allowed  to  sag  a  little  into 
the  dish,  so  that  it  may  be  bathed  on  both  sides  by  the  silver  solution. 
It  is  now  to  be  carefully  washed  with  distilled  water  to  remove  any 
albuminous  substance  or  blood — which  would  cause  a  granular  precipi- 
tate of  silver  albuminate  on  the  surface — and  the  dish  then  filled  with 
an  aqueous  solution  of  nitrate  of  silver,  1  to  500.  The  dish  should  be 
gently  shaken  at  frequent  intervals,  so  as  to  bring  fresh  portions  of  the 
solution  into  contact  with  the  membrane,  and  after  from  twenty 
minutes  to  half  an  hour  the  tissue  will  be  seen  to  have  become  cloudy 
or  milky. 

The  silver  is  now  poured  off  and  the  membrane  carefully  washed 
with  pure  water.  The  cells  will  have  been  fixed  by  the  silver,  so  that 
the  membrane  may  be  removed  without  further  danger  of  disturbing 
the  relations  of  the  cells,  and  laid  in  a  dish  containing  water  to  which 
one-third  its  bulk  of  alcohol  has  been  added.  It  is  now  exposed  to  the 
sunlight,  and  after  from  a  few  minutes  to  half  an  hour — sometimes 
longer,  depending  upon  the  intensity  of  the  light — the  tissue  will  be 
seen  to  have  assumed  a  brown  color.  The  specimens  are  now  trans- 
ferred to  dilute  hematoxylin,  where  they  remain  until  sufficiently 
stained,  and  finally  small  bits  are  cut  off,  spread  smoothly  on  a  slide, 
and  mounted  in  glycerine.  The  mesentery  will  be  seen  to  consist  of 
a  thin  membrane  of  fibrillar  connective  tissue  with  a  few  delicate 
elastic  fibres,  and  here  and  there  small  blood-vessels,  the  whole 
covered  with  the  delicate  mosaic  of  endothelial  cells.  The  out- 
lines of  the  endothelium  on  both  the  upper  and  under  sides  of  the 
membrane  may  be  brought  successively  into  view  by  careful  focus- 
sing. 

Omentum. — A  portion  of  the  omentum  of  the  dog  should  be  treated 
with  silver  in  the  way  just  directed  for  the  mesentery,  and  also  stained 
with  hematoxylin  and  mounted  in  glycerine.  The  omentum  of  the 
dog,  as  of  man,  consists  of  an  irregular  meshed  net,  whose  trabecule 
are  made  up  of  fascicles  of  fibrillar  connective  tissue  of  varying  thick- 
ness, the  broader  containing  blood-  and  lymph-vessels  and  fat-cells, 
the  narrower  consisting  of  single  bundles  offibrille — all  being  alike 
covered  with  a  single  layer  of  endothelium.  Here  and  there  are  seen 
groups  of  irregularly  polygonal  granular  cells,  which  are  connected 
with  the  lymphatic  apparatus.  In  many  parts  the  endothelial  cells 
are  seen  in  profile,  when  their  nuclei  appear  lenticular  in  form,  usually 
projecting  above  the  general  level  of  the  cell-body,  which  is  itself  in 
most  cases  thicker  in  the  vicinity  of  the  nucleus  than  at  the  points  of 
junction  with  adjoining  cells.  The  specimens  of  both  omentum  and 
mesentery  may  be  preserved  in  the  usual  way;  but  silver  preparations, 
like  the  gold,  do  not  preserve  their  first  clearness  very  long,  unless 
carefully  kept  from  the  light. 

Stomata. — Considering  the  serous  cavities  as  spaces  in  the  con- 
nective tissue   lined  with   modified  connective-tissue   cells,  it  will  be 


28  NORMAL  HISTOLOGY. 

seen  that,  although  unlike  in  general  character,  they  are  quite  analo- 
gous with  the  smaller  spaces,  containing  sometimes  only  one  cell, 
which  are  found  everywhere  in  the  connective  tissue,  and  which  were 
studied  in  detail  in  the  cornea.  This  analogy  becomes  still  more  evi- 
dent when  we  consider  their  relations  to  the  lymphatic  vessels.  In 
considering  the  connective-tissue  spaces  of  the  cornea,  it  was  said  that 
although  serving  as  lymph-passages,  it  had  never  been  possible  as  yet 
to  demonstrate  their  exact  mode  of  communication  with  the  lymphatic 
vessels.  In  the  large  lymph-spaces,  such  as  the  peritoneal  cavity, 
however,  the  mode  of  communication  has  been  quite  definitely  ascer- 
tained. In  those  parts  of  the  walls  of  the  peritoneum  which  are  most 
abundantly  supplied  with  lymphatic  vessels — for  example,  in  the  cen- 
tral tendon  of  the  diaphragm — numerous  small  openings,  called  stoma- 
ta,  exist,  leading  from  the  peritoneal  cavity  directly  into  the  ves- 
sels, and  these  openings  are  readily  recognized  by  a  peculiar  arrange- 
ment of  the  endothelial  cells  in  their  vicinity.  The  stomata  may  be 
more  easily  studied,  however,  in  the  frog  than  in  the  warm-blooded 
animals.  In  this  animal  there  lies  directly  in  front  of  the  vertebral 
column,  and  extending  the  whole  length  of  the  trunk,  a  large  lym- 
phatic vessel  or  cistern,  which  is  separated  from  the  peritoneal  cavity 
by  a  thin  connective-tissue  septum  covered  on  both  sides  by  endo- 
thelial cells.  In  this  septum  are  found  a  great  number  of  stomata. 
They  are  readily  demonstrated  by  the  following  procedure  :  the 
spinal  cord  of  a  frog  being  broken  up  with  a  needle,  the  skin  is  cut 
around  just  behind  the  front  legs,  and  entirely  stripped  from  the  body. 
The  peritoneal  cavity  is  opened  by  an  incision  reaching  from  the 
symphysis  to  the  sternum,  and  the  entire  visceral  contents,  with  the 
exception  of  the  kidneys,  removed — care  being  taken  to  cut  the  mes- 
entery as  far  as  possible  from  its  posterior  attachment,  so  as  not  to 
injure  the  septum  of  the  lymphatic  sac.  The  hind  legs  are  now  tied 
together  by  a  thread  ;  the  anterior  part  of  the  body  cut  off  just  be- 
hind the  front  legs  ;  the  remaining  part  washed  with  pure  water,  to 
remove  blood,  and  suspended  in  a  solution  of  nitrate  of  silver,  i  to 
500.  Here  it  should  be  frequently  agitated  to  bring  fresh  portions  of 
the  solution  into  contact  with  the  membrane,  and  after  from  twenty 
minutes  to  half  an  hour,  when  the  tissue  has  become  milky,  the  whole 
should  be  removed  and  washed,  and  laid  in  a  shallow  dish  containing 
pure  water.  A  thin  white  membrane  will  now  be  seen  floating  on 
each  side  of  the  kidneys  ;  this  is  the  anterior  wall  of  the  lymph-sac. 
After  exposure  to  the  light,  until  the  membrane  becomes  brown,  it 
should  be  snipped  out  with  the  scissors,  stained  with  haematoxylin, 
and  mounted  in  glycerine  with  the  peritoneal  surface  uppermost. 

The  general  surface  is  seen  to  be  covered  with  polygonal  endo- 
thelium, while  scattered  here  and  there  over  the  specimen  are  tiny 
openings  around  which  the  endothelial  cells  are  arranged  in  a  more  or 
less  radiate  manner.  These  cells  surrounding  the  stomata  are  usually 
considerably  elongated  ;  and  whereas  over  the  general  surface  the 
nuclei  are  situated  near  the  centre  of  the  cells,  here  they  lie  near  the 
edge  which  borders  the  opening.  By  careful  focussing  one  can  readi- 
ly distinguish  the  silver  markings  which  outline  the  cells  on  the  under 


CONNECTIVE    TISSUE.  29 

side  of  the  specimen,  those  lining  the  lymph-sac,  as  well  as  the  con- 
nective-tissue fibres  and  cells  above  them. 

The  cells  on  the  side  toward  the  lymph-sac  often  extend  inward 
over  the  opening,  so  as  nearly  or  entirely  to  close  it :  it  would  seem, 
indeed,  as  if  the  stomata  were  rather  valvular  than  permanently 
patulous  openings. 

One  sometimes  sees,  here  and  there,  black  irregular  spots  between 
the  endothelium,  which  should  not  be  mistaken  for  stomata,  as  they 
are  only  irregular  deposits  of  the  albuminate  of  silver  in  the  cement- 
ing substance. 


CHAPTER    III. 

EMBRYONAL    AND    MUCOUS     TISSUE  — FAT    TISSUE  — 
RETICULAR   TISSUE. 

Embryonal  and  Mucous  Tissue. 

At  a  certain  period  of  embryonic  life,  those  parts  of  the  body  which 
are  destined  finally  to  become  fibrillar  connective  tissue  consist  almost 
entirely  of  small  spheroidal  cells — the  intercellular  substance  being 
absent,  or  consisting  only  of  a  very  small  quantity  of  fluid  lying  be- 
tween and  bathing  the  cells.  As  the  process  of  development  goes  on, 
some  of  the  cells  retain  their  original  spheroidal  form,  while  others 
change  their  character,  becoming  elongated  and  fusiform,  often  termi- 
nating at  their  extremities  in  delicate  single  or  branching  processes ; 
others  become  flattened  and  assume  irregular  shapes,  sending  off 
branching  processes  by  which  they  are  joined  to  one  another.  Hand  in 
hand  with  this  change  in  the  cells  there  occurs  an  accumulation  of 
intercellular  material,  which  is  at  first  fluid,  and  later  presents  the  ap- 
pearance of  a  homogeneous  gelatinoid  substance.  Then  within  the 
gelatinoid  intercellular  substance  appear  fine  fibrillse,  which  become 
more  and  more  abundant,  arranging  themselves  now  in  bundles,  and 
again  to  form  irregular  net-works.  The  cells  approach  more  and  more 
closely  to  the  type  of  the  adult  connective-tissue  cells,  as  development 
goes  on  ;  the  intercellular  substance  loses  its  soft  gelatinoid  character, 
and  is  finally  replaced  by  the  fibrillated  and  elastic  fibres  with  which 
we  are  already  familiar.  The  process  of  development  is  a  very  grad- 
ual one,  and  although  the  younger  forms  of  embryonal  connective 
tissue  and  the  adult  connective  tissue  are  distinct  enough,  we  are  yet 
unable  to  separate  them  sharply,  since  they  merge  so  gradually  into 
one  another.  In  general,  however,  simply  as  a  matter  of  convenience, 
we  call  connective  tissue,  which  is  almost  entirely  made  up  of  sphe- 
roidal, spindle-shaped,  or  flattened  cells,  in  which  little  accumulation 
and  little  differentiation  of  the  intercellular  substance  has  occurred, 
embryonal  tissue  ;  while  to  that  older  form,  which  consists  of  variously 
shaped,  spheroidal,  flat,  branching  and  anastomosing  cells,  with  a 
gelatinoid,  homogeneous,  or  partially  fibrillated  intercellular  substance, 
the  term  mucous  tissue  is  usually  applied.  The  name  mucous  tissue 
was  given  to  this  form  of  young  tissue,  because  the  soft  gelatinoid  in- 
tercellular substance  was  found  to  contain  a  certain  amount  oi  murine, 
which  may  be  thrown  down  in  the  form  of  a  whitish,  often  stringy 
precipitate,  by  the  addition  of  acetic  acid.  At  present,  however,  tis- 
sues presenting  the   above-mentioned   characters   are   usually  called 


EMBRYONAL  AND    MUCOUS   TISSUE,  ETC.  3 1 

mucous  tissues,  whether  the  intercellular  substance  contains  mucine 
or  not.  Mucous  tissue  is  not  found  in  the  human  adult  under  normal 
conditions,  but  frequently  occurs  as  a  pathological  production. 


PRACTICAL    STUDY. 

Subcutaneous  Tissue  of  Embryo. — Embryonal  connective  tissue 
may  be  studied  in  the  subcutaneous  tissue  of  any  young  mammalian 
embryo.  The  animal  having  been  placed  for  a  few  days  in  Muller's 
fluid,  bits  of  the  subcutaneous  tissue  are  torn  off  from  the  abdominal 
wall  with  fine  forceps,  stained  first  with  hasmatoxylin,  and  then  with 
eosin,  carefully  teased  in  glycerine,  and  covered. 

Umbilical  Cord. — In  the  umbilical  cord  we  have  a  typical  example 
of  mucous  tissue.  The  cord  of  any  young  mammal,  as  the  pig,  is  to 
be  put  into  Muller's  fluid,  where  it  remains  for  a  week.  It  is  then 
washed  and  transferred  to  alcohol.  After  twenty-four  hours  it  may  be 
embedded  between  two  bits  of  hardened  liver,  and  transverse  sections 
made  from  it.  The  sections  are  stained  with  hematoxylin  and  eosin, 
and  studied  in  glycerine,  in  which  they  may  be  preserved  ;  or  the  water 
maybe  removed  and  the  specimens  mounted  in  Canada  balsam.  The 
surface  of  the  cord  is  seen  to  be  covered  with  several  layers  of  flattened 
epithelium,  and  the  three  large  blood-vessels  are  seen  in  transverse 
sections.  The  amount  of  fibrillation  present  in  the  intercellular  sub- 
stance depends  upon  the  age  of  the  embryo. 

Fat  Tissue. 

Fat-tissue  is  a  modified  form  of  connective  tissue,  in  which  the 
intercellular  substance  is  present  in  proportionally  small  amount,  a 
large  part  of  the  protoplasm  of  the  cells  being  replaced  by  fat,  which 
crowds  the  remaining  part  together  with  the  nucleus  to  one  side  of  the 
cell,  nearly  or  entirely  concealing  both.  The  fat-cells  thus  formed 
are  arranged  in  clusters  or  lobules,  enclosed  by  fibrillar  connective 
tissue,  which  sends  into  the  lobules  and  between  the  cells  broader 
and  narrower  bundles,  which  serve  to  support  the  cells  and  carry  the 
blood  and  lymphatic  vessels,  etc.  Owing  to  the  pressure  to  which 
the  fat-cells  are  subjected,  they  usually  assume  in  the  adult  animal  a 
polyhedral  form.  When  fresh  fat  is  examined  by  teasing  bits  of  the 
tissue  from  a  recently  killed  animal,  in  \  per  cent,  salt  solution,  little 
else  is  seen  than  a  congeries  of  more  or  less  globular  or  polyhedral 
masses  of  fat-cells,  with  larger  and  smaller  fat  droplets.  The  fat  is  char- 
racterized  by  its  great  refractive  power ;  by  the  dark  contour  of  its 
globules  by  transmitted,  and  their  yellowish  silvery  lustre  by  reflected 
light ;  by  its  solubility  in  alcohol  and  ether,  and  by  its  assumption  of 
a  deep  black  color  on  treatment  with  osmic  acid.  These  fat-cells  are 
enclosed  by  a  membrane,  though  this  is  usually  invisible  in  fresh  fat, 
and  here  and  there  the  nuclei  are  seen  crowded  to  one  side.  The 
connective  tissue  which  encloses  the  lobules  is  seen  in  picked  speci- 
mens in  broken  masses  and  shreds,  scattered  through  the  preparation. 
Sometimes  the  fat  is  crystallized  within  the  cells,  when  it  appears  in 


32  NORMAL   HISTOLOGY. 

the  form  of  masses  of  radiating  needle-like  crystals.  The  methods  by 
which  we  can  demonstrate  the  presence  of  cell  protoplasm  and  nuclei 
in  adult  fat-tissue,  and  their  relations  to  the  cell-membrane  and  the 
fat,  will  be  given  below. 

In  order  to  understand  clearly  the  nature  of  adult  fat-tissue,  it  is 
necessary  to  study  it  during  the  process  of  development.  At  an  early 
period  of  life,  those  parts  of  the  body  which  are  finally  to  become  fat- 
tissue  possess  the  character  of  mucous  tissue  with  a  more  or  less  fibril- 
lated  intercellular  substance.  The  first  change  which  we  notice  in 
the  cells  as  the  transformation  into  fat-tissue  commences,  is  the  ap- 
pearance in  the  cell-body  of  small  shining  particles.  These  particles, 
of  which  there  may  be  many,  become  gradually  larger,  until  they  pre- 
sent the  form  and  character  of  distinct  droplets  of  fat.  As  these  drop- 
lets increase  in  size,  they  coalesce,  forming  one  or  more  drops,  which 
presently  become  large  enough  to  crowd  the  nucleus  to  one  side. 
The  growing  drops  finally  unite  into  one  large  drop,  which  at  length 
becomes  so  large  as  to  occupy  nearly  the  whole  of  the  cell,  leaving 
only  a  thin  crescent  of  protoplasm  and  a  squeezed  and  distorted 
nucleus  crowded  up  against  the  cell-membrane.  At  last  we  can  no 
longer  see,  without  special  modes  of  preparation,  any  trace  of  cell 
protoplasm,  and  only  the  deformed  remnant  of  the  nucleus.  This 
process  is  called  fatty  infiltration.  In  those  parts  of  the  body  where 
the  fat  is  invariably  found,  this  change  in  the  cells  occurs  in  the  vicin- 
ity of  little  tufts  of  capillary  blood-vessels,  so  that  at  one  period  the 
forming  fat  is  seen  lying  in  scattered  clusters  in  the  meshes  of  distinct 
groups  of  blood-capillaries.  It  is  these  clusters  of  fat-cells,  with  their 
accompanying  blood-vessels,  which  determine,  when  the  fat  is  fully 
formed,  the  lobular  character  of  the  tissue.  In  many  parts  of  the 
body  and  under  varying  conditions,  sometimes  physiological,  some- 
times pathological,  there  is  an  accumulation  of  fat  in  the  protoplasm 
of  cells ;  but  it  is,  for  the  most  part,  temporary,  and  the  fat-cells  have 
no  definite  grouping  in  lobules  and  about  the  blood-vessels  in  the  way 
above  described  for  the  permanent  fat. 


PRACTICAL    STUDY. 

Fresh  Fat  from  Rabbit. — A  bit  of  subcutaneous  fat  from  the 
freshly  killed  rabbit  should  be  teased  on  a  slide  in  salt  solution,  and 
studied  without  staining. 

Fat  Tissue  treated  with  Nitrate  of  Silver. — If  a  bit  of  skin  be 
reflected  back  from  the  abdominal  wall  of  a  recently  killed  rabbit, 
and  an  interstitial  injection  of  a  solution  of  nitrate  of  silver,  i  to 
1,000,  be  made  into  one  of  the  fat-lobules  which  are  exposed  in  the 
subcutaneous  tissue,  and  a  small  bit  of  the  oedematous  fat  snipped 
off  at  once  and  carefully  teased  on  a  slide  without  the  addition  of 
other  fluid,  the  various  parts  of  the  fat-cells  can  be  readily  seen. 

In  some  way  which  we  do  not  yet  understand,  the  silver  causes 
the  narrow  rim  of  cell  protoplasm  to  swell,  separating  the  membrane 
and  the  nucleus  from  the  fat  in  such  a  way  as  to  make  a  complete 
demonstration  of  the  structure  of  the  cell. 


EMBRYONAL  AND    MUCOUS   TISSUE,  ETC.  33 

Cross-seciion  of  Adult  Fat.— A.  bit  of  fat-tissue  from  the  adult 
human  subject  should  be  hardened  in  alcohol,  by  which  the  fat  will  be 
for  the  most  part  dissolved  out  of  the  cells,  leaving  them  filled  with 
the  alcohol.  Thin  sections  are  made  and  stained  with  hematoxylin. 
These  preparations  show  the  relations  of  the  cells  to  one  another,  and 
the  lobular  character  of  the  fat-tissue.  If  the  blood-vessels  of  the 
part  from  which  the  tissue  is  taken  have  been  previously  filled  with 
some  colored  injection,  the  relations  of  the  lobules  to  the  blood- 
vessels will  be  well  shown. 

Uninjected  specimens  may  be  preserved  in  glycerine;  injected 
specimens  may  be  stained  with  hematoxylin  and  eosin.  and  mounted 
in  Canada  balsam. 

Developing  Fat  from  Young  Animal— -Bits  of  the  omentum  or 
mesentery  of  a  nearly  mature  fcetal  or  a  new-born  rabbit  should  be  snip- 
ped off  and  carefully  spread  out  on  a  slide  with  the  addition  of  salt 
solution.  Cells,  singly  and  in  clusters,  will  be  seen  which  present 
various  stages  of  the  above  described  fatty  infiltration.  For  permanent 
preservation  the  specimens  may  be  laid,  for  an  hour,  in  dilute  alco- 
hol— i  alcohol,  &  water — then  stained  with  eosin  and  mounted  in 
glycerine. 

Reticular  Connective  Tissue. 

This  tissue  forms  a  large  part  of  the  supporting  framework  of  the 
lymphatic  glands,  and  is  found,  in  somewhat  modified  form,  in  other 
parts  of  the  body.  It  consists  of  extremely  fine  fibres,  which,  running 
in  all  directions,  cross  and  join  one  another  at  frequent  intervals, 
forming  a  finely  meshed  net-work.  This  net-work  of  fibres  is  not  flat- 
tened to  form  a  membrane,  but  extends  in  all  directions,  like  the  tra- 
becule of  a  sponge.  Irregularly  scattered  over  the  fibres  are  flattened 
nucleated  cells,  having  the  character  of  endothelium,  which  sometimes 
lie  at  the  points  of  intersection  of  the  fibres,  sometimes  along  their 
sides,  enwrapping  them  with  their  transparent  bodies.  When  the  cells 
are  in  situ  upon  the  fibres,  the  whole  presents  the  appearance  of  a 
mass  of  anastomosing,  branched,  or  spindle-shaped  cells  ;  and,  as 
such,  the  reticular  connective  tissue  has  until  recently  been  regarded — 
erroneously  however,  as  our  practical  study  will  show.  The  meshes  of 
the  reticular  tissue  are  loosely  filled,  in  the  lymphatic  glands,  with 
small  round  cells — lymph-cells — which,  however,  seem  to  have  no 
direct  connection  with  the  tissue  we  are  studying,  and  may  be  easily 
removed  by  the  manipulations  presently  to  be  described. 

practical  study. 

Lymphatic  Gland  of  Dog  treated  with  Osmic  Acid. — From  a  re- 
cently killed  dog  one  of  the  mesenteric  glands  should  be  removed, 
and  a  hypodermic  syringe  being  partially  filled  with  a  one  per  cent, 
solution  of  osmic  acid,  the  canula  is  thrust  well  into  the  gland  and  the 
acid  slowly  injected  till  the  organ  becomes  somewhat  tense;  the 
canula  is  then  withdrawn  and  the  gland  dropped  into  strong  alcohol. 
After  a  few  days  the  gland  will  be  sufficiently  hard  to  permit  the  pre- 
'3 


34  NORMAL   HISTOLOGY. 

paration  of  thin  sections  with  a  razor.  The  sections  should  be  carefully 
shaken  in  a  test-tube  one-third  filled  with  water,  to  remove  the  lymph- 
cells,  which  lie  in  the  meshes  of  the  reticular  tissue  and  conceal  it. 
They  are  then  stained  with  hematoxylin,  and  studied  and  preserved  in 
glycerine.  The  object  of  the  osmic  acid  injection  is  to  harden  the 
flat  cells  which  lie  upon  the  fibres,  and  fix  them  firmly  in  place.  The 
tissue  really  looks,  after  this  mode  of  preparation,  like  a  mass  of 
branching  anastomosing  cells. 

Lymphatic  Gland  of  Dog  treated  with  Picric  Acid. — By  the  fol- 
lowing mode  of  preparation,  the  cells  which  lie  upon  the  fibres  are 
loosened  from  the  latter,  and  after  their  separation,  by  shaking  the 
specimens  in  water,  we  see  simply  a  mass  of  intersecting  and  anasto- 
mosing fibres. 

Another  gland  of  the  dog  should  be  put  into  a  saturated  solution 
of  picric  acid,  and  allowed  to  remain  twenty-four  hours  (not  longer), 
then  transferred  to  strong  alcohol  till  it  is  hard  enough  to  cut.  Thin 
sections  are  shaken,  as  before,  in  water  and  stained.  The  shaking 
should  be  carefully  and  efficiently  done,  or  many  of  the  cells  will  re- 
main clinging  to  the  fibres.  Indeed,  in  the  best  preparations,  this  is 
apt  to  be  the  case  in  some  parts  of  the  specimen  •  but  it  is  not  difficult 
to  free  sufficiently  large  areas  of  the  section  to  convince  ones'-self  of 
the  truth  of  the  above  view  of  the  nature  of  the  reticular  connective 
tissue. 


CHAPTER  IV. 

CARTILAGE— BONE— TEETH. 

Cartilage. 

Cartilage  consists,  like  the  other  members  of  the  connective-tissue 
group,  of  cells  and  intercellular  substance.  There  is  nothing  charac- 
teristic, however,  in  the  form  of  the  cartilage-cells.  It  is  in  the 
peculiar  nature  of  the  intercellular  substance  and  the  relations  which 
the  cells  bear  to  it,  that  we  find  the  distinctive  features  of  this  form  of 
connective  tissue.  The  cartilage-cells  are  spheroidal,  flattened,  or 
angular  in  form ;  the  cell-body  is  transparent,  and  often  contains  tiny 
droplets  of  fat,  and  sometimes  pigment-granules.  The  cells  have  one, 
or  sometimes  two  sharply  defined  nuclei,  which  are  granular  and  often 
seem  to  contain  an  irregular  network  of  some  more  strongly  refractive 
substance.  Round  each  cartilage-cell,  in  the  adult  animal,  and  closely 
enclosing  it,  is  a  homogeneous  envelope  called  the  capsule.  The  sub- 
stance forming  this  capsule  is  identical  with  the  intercellular  substance 
of  hyaline  cartilage,  presently  to  be  described.  The  cartilage-cells 
very  readily  lose  their  normal  form  and  relation  to  the  capsule  by  the 
application  of  a  great  variety  of  substances,  even  by  slight  exposure  to 
the  air.  The  most  common  change  which  is  noticed  in  them  is  a 
rapid  shrinkage,  such  as  occurs  when  cartilage  is  exposed  to  the  air  or 
treated  with  strong  salt  solutions,  or  any  substance  which  extracts 
water  from  the  tissues.  Under  these  circumstances  the  cell  becomes 
more  coarsely  granular,  it  shrinks  away  from  the  capsule  at  certain 
points,  giving  the  edge  of  the  cell  a  festooned  appearance  ;  sometimes 
large,  clear  spheroidal  spaces,  called  vacuoles,  appear  in  the  cell-body, 
and  finally  the  cell  shrinks  to  a  shapeless  mass  in  the  centre  or  at  one 
side  of  the  cavity,  or  retains  its  connection  with  the  capsule  by  one  or 
more  narrow  irregular  projections  from  the  shrunken  mass. 

The  basement  or  intercellular  substance  is  not  the  same  in  all  carti- 
lages, and,  according  to  the  differences  in  its  nature,  cartilage  is  divided 
into  hyaline  cartilage,  fibro-cartilage,  ftbro-elastic  cartilage.  In  hya- 
line cartilage  the  intercellular  substance  is  homogeneous  and  transpar- 
ent in  thin  layers,  somewhat  opalescent  in  thicker  masses  ;  it  is  of  firm 
consistence,  cutting  readily  with  the  knife,  and  contains  at  tolerably 
regular  intervals  variously  shaped  cavities  in  which  the  cells  lie,  exactly 
filling  them.  The  layer  of  basement  substance  which  immediately  sur- 
rounds the  cells  possesses  slightly  different  refractive  power,  and  it  is  this 
layer  which  constitutes  the  capsule  above  described.  The  cell-spaces 
or  cavities  in  hyaline  cartilage  do  not  by  the  ordinary  modes  of  prepa- 


36  NORMAL  HISTOLOGY. 

ration  appear  to  be  connected  by  lymph-channels,  as  are  the  cell-spaces 
in  other  varieties  of  connective  tissue,  yet  the  rapid  passage  of  fluids 
through  the  tissue  under  certain  circumstances,  together  with  some 
recent  microscopical  observations,  render  it  extremely  probable  that 
such  communications  do  exist,  though  we  are  not  at  present  able  to 
demonstrate  them  with  certainty.  Although  by  the  ordinary  modes  of 
preparation  the  basement  substance  of  hyaline  cartilage  appears  quite 
homogeneous,  certain  changes  which  it  undergoes  under  pathological 
conditions,  or  by  the  use  of  certain  macerating  or  digesting  fluids,  lead 
us  to  believe  that  it  really  contains  a  groundwork  of  delicate  fibrillae. 
The  basement  substance  of  hyaline  cartilage  differs  chemically  from 
the  basement  substance  of  other  members  of  the  connective-tissue 
group,  yet  recent  researches  have  thrown  serious  doubt  upon  the  view 
formerly  held,  i.  <?.,  that  cartilage  gave,  on  boiling,  a  peculiar  and 
characteristic  substance  called  chondrine,  it  having  been  shown  that 
the  so-called  chondrine  is  not  a  pure  chemical  substance,  but  a  mix- 
ture of  mucine  and  various  albuminoid  materials.  Hyaline  cartilage  is 
found,  in  the  adult,  covering  the  ends  of  the  bones,  in  the  joints,  and  in 
most  of  the  laryngeal  cartilages.  The  tracheal,  bronchial,  and  costal 
cartilages  are  also  of  this  variety.  Fibro-cartilage  differs  from  hyaline 
cartilage  in  having  a  distinctly  nbrillated  intercellular  substance.  This 
form  of  cartilage  is  found  in  the  intervertebral  cartilages,  in  the  menis- 
cuses  of  certain  joints,  and  at  certain  points  where  the  ligaments  are 
inserted  into  the  cartilaginous  extremities  of  bones.  In  certain  other 
cartilages,  such  as  the  epiglottis,  some  of  the  small  cartilages  of  the 
larynx,  and  the  cartilage  of  the  pinna  of  the  ear,  the  intercellular  sub- 
stance contains,  in  addition  to  a  few  fibres,  such  as  are  seen  in  fibro- 
cartilage,  elastic  tissue,  either  in  the  form  of  fibres  or  in  fine  granules. 
Such  cartilage  is  called  fibro-elastic  cartilage.  In  both  fibro-  and  fibro- 
elastic  cartilage  the  cells  are  identical  in  character  with  those  of  hya- 
line cartilage.  Except  at  the  free  surfaces,  which  it  presents  in  the 
joints,  cartilage  is  surrounded  by  a  layer  of  nbrillated  connective  tissue 
of  varying  thickness,  called  the  perichondrium,  in  which  are  found  the 
blood-vessels  which  supply  nutritive  material  to  the  non-vascular  car- 
tilage within.  At  the  surface  of  many  cartilaginous  masses  the  cells 
are  very  much  crowded  together,  and  flattened  in  a  plane  parallel  to 
the  surface. 

PRACTICAL    STUDY.  b. 

Hyaline  Cartilage  from  Femur  of  Frog. — The  head  of  the  femur 
of  a  recently  killed  animal  being  exposed,  a  thin  slice  of  cartilage  is 
shaved  off  from  the  surface  with  a  razor,  so  as  to  leave  a  flat  surface 
from  which  a  thin  section  should  be  cut  and  immersed  in  a  drop  of 
saturated  solution  of  picric  acid  on  a  slide,  and  covered  and  surrounded 
at  once  by  a  rim  of  asphalt  varnish  before  the  acid  solution  commences 
to  evaporate. 

Most  of  the  preservative  fluids  cause  a  rapid  shrinkage  of  the  carti- 
lage-cells. Picric  acid,  however,  preserves  the  cells  in  nearly  normal 
form  for  a  long  time ;  but  even  in  this  they  shrink  somewhat  after  a 
time,  or  become  coarsely  granular. 


CARTILAGE— BONE— TEETH.  37 

In  such  thin  sections,  here  and  there  cavities  are  seen  from  which 
the  cells  have  fallen  out ;  they  may  be  filled  with  the  preservative 
fluid  or  with  bubbles  of  air. 

Shrinkage  of  Cartilage  Cells  by  Strong  Salt  Solution. — Since  in 
most  of  the  specimens  containing  cartilage,  which  we  shall  study  in 
subsequent  lessons,  the  cells  will  have  been  considerably  shrunken  by 
the  preservative  fluids,  it  is  desirable  that  we  should  acquaint  our- 
selves with  the  process  of  shrinkage  here,  so  that  we  may  be  able  to 
understand  the  varied  forms  which  such  cells  present. 

To  do  this  we  have  simply  to  prepare  a  thin  section  of  hyaline  car- 
tilage as  above,  and  immerse  it  at  once  in  one-half  per  cent,  solution 
of  salt.  When  a  good  field  for  observation  has  been  selected,  a  drop 
of  ten  per  cent,  salt  solution  should  be  put  at  the  edge  of  the  cover- 
glass,  the  other  fluid  being  drawn  out  by  a  bit  of  blotting-paper  placed 
at  the  opposite  side.  In  this  way  the  process  of  shrinkage  and  forma- 
tion of  vacuoles  may  be  readily  observed. 

Fibro-cartilage  may  be  studied  in  thin  sections  from  the  inter- 
vertebral cartilages  of  man  or  any  of  the  domestic  animals,  or  from 
the  head  of  the  femur,  through  the  insertion  of  the  ligamentum  teres, 
parallel  with  the  course  of  its  fibres.  The  tissues  should  be  laid  for  a 
few  days  in  alcohol  before  cutting.  The  sections  are  stained  with 
picro-carmine  and  mounted  in  glycerine. 

Fibro-elastic  Cartilage. — This  is  commonly  studied  in  the  epi- 
glottis of  man  or  the  lower  animals,  which  has  been  preserved  in  alco- 
hol. Thin  sections  are  stained  with  picro-carmine,  which  colors  the 
cells  red,  and  the  elastic  granules  and  fibres  yellow.  They  are  mounted 
and  preserved  in  glycerine.  The  cells  in  both  fibro-  and  fibro-elastic 
cartilage  are,  of  course,  by  the  above  modes  of  preparation,  shrunken 
and  deformed. 

Boxe. 

In  studying  bone,  we  have  to  consider:  1,  the  hard  substance,  or 
bone-tissue  proper  ;  2,  the  connective-tissue  envelope  which  surrounds 
the  bone — the  periosteum  ;  and  3,  the  marroiu  contained  in  the  central 
cavities  or  spaces  within. 

1.  The  most  striking  feature  of  bone-tissue  proper  is  its  firmness 
and  hardness,  which  is  due  to  the  presence  of  certain  inorganic  salts 
of  lime.  If  a  bone  be  soaked  for  some  time  in  dilute  solutions  of 
hydrochloric,  or  nitric,  or  chromic  acid,  or  in  a  saturated  solution  of 
picric  acid,  the  lime  salts  are  entirely  dissolved,  and  there  is  left  be- 
hind a  substance  which  retains,  for  the  most  part,  both  in  general 
form  and  in  minute  structure,  all  the  essential  features  of  the  original 
bone.  There  is  a  basement  substance,  presenting  many  of  the  optical 
characters  of  hyaline  cartilage,  and  lying  in  tiny,  branching  spaces, 
in  the  basement  substance,  are  flat,  nucleated  cells. 

The  lime  salts  are  deposited  in  the  intercellular  substance  in  such  an 
extremely  minute  state  of  division  as  to  be  invisible,  even  with  high 
powers  of  the  microscope.  The  elongated  and  flattened  cell-spaces  of 
bone  are  frequently  called  lacunce,  and  the  numerous  fine,  branching,  in- 


38  NORMAL   HISTOLOGY. 

tercommunicating  channels  which  pass  off  from  them  in  all  directions 
and  open  into  the  marrow  cavities,  or  into  the  passages  for  the  blood- 
vessels, are  called  canaliculi. 

The  bone-cells,  or  bone-corpuscles,  as  they  were  formerly  called, 
are,  in  adults,  thin,  flat  cells,  with  spheroidal  or  oval  projecting  nuclei. 
It  was  formerly  believed  that  they  sent  fine  branching  processes  off 
into  the  ramifications  of  the  canaliculi.  Recent  investigations,  how- 
ever, have  thrown  great  doubt  upon  the  existence  of  at  least  such 
numerous  processes  as  were  formerly  believed  to  exist — it  having 
been  shown  that  some  at  least,  if  not  all,  of  the  supposed  processes 
are  really  portions  of  the  intercellular  substance  lining  the  lacunae 
and  canaliculi.  In  young  bone  the  cells  are  not  flat,  but  spherical  or 
ovoid  al. 

We  distinguish  two  kinds  of  bone-tissue,  spongy  and  compact. 

In  spongy  bone-tissue,  which  is  found  in  abundance  in  the  epiphyses 
of  the  long  bones,  the  hard  substance,  or  bone  proper,  is  arranged  in 
the  form  of  thin  plates,  which  are  grouped  together  so  as  to  enclose 
tiny,  irregularly-shaped  cavities,  filled  with  marrow  tissue.  In  these 
thin  plates  of  spongy  bone  the  cells  lie  irregularly  scattered  through 
the  intercellular  substance,  which  is  homogeneous. 

In  compact  bone,  such  as  is  found  in  the  diaphyses  of  the  long  bones, 
the  intercellular  substance  is  arranged  in  layers,  or  lamellae,  in  and  be- 
tween which  lie  the  cells.  The  lamellar  arrangement  is  best  seen  in 
transverse  sections  from  the  diaphyses  of  the  long  bones.  If  we  look 
at  a  thin  cross-section  of  such  a  bone  with  a  low  magnifying  power,  we 
notice  numerous  round  or  ovoid,  or  irregular-shaped  openings,  of 
varying  size,  and  around  these  are  grouped  several  thin  concentric 
layers  of  basement  substance,  in  and  between  which  lie  the  ce]ls,  flat- 
terled  in  the  plane  of  the  lamellae. 

These  sets  of  concentric  lamellae  are  called  special  or  Haversian 
systems  of  lamella.  Sometimes  these  Haversian  systems  of  lamellae 
lie  closely  crowded  together,  and  again  they  lie  at  varying  distances 
from  one  another.  In  the  latter  case  the  intervening  space  is  filled 
up  by  other  and  irregular  sets  of  lamellae,  which  do  not  correspond 
with  the  Haversian  lamellae,  but  pass  off  obliquely  in  various  direc- 
tions. These  are  called,  from  their  position  relative  to  the  Haversian 
system,  intermediate  lamellae.  Finally,  at  the  surface  of  the  bone  be- 
neath the  periosteum,  and  sometimes  at  the  inner  surface  adjacent  to 
the  medullary  cavity,  are  seen  a  series  of  lamellae  which  lie  parallel  to 
the  surfaces  of  the  bone,  which  are  called  general  or  circumferential 
lamellae.  If  we  make  a  longitudinal  section  of  a  long  bone  we  find 
that  it  is  traversed  by  a  number  of  more  or  less  longitudinally  arranged, 
branching  and  communicating  canals,  of  varying  size,  in  which  lie  the 
blood-vessels.  It  is  around  these  canals,  called  Haversian  ca?zals,  that 
the  Haversian  lamellae  are  grouped,  and  the  variously  shaped  openings 
which  are  seen  in  the  transverse  sections  are  the  transverse  sections  of 
these  blood-vessel,  or  Haversian  canals.  Within  the  Haversian  canals, 
when  they  are  not  entirely  filled  with  the  blood-vessels,  we  find  the  latter 
enclosed  in  a  tissue  identical  with  that  .filling  the  medullary  cavity,  and 
presently  to  be  described  as  marrow.     In  the  flat  and  irregular-shaped 


CARTILAGE — BONE — TEETH.  39 

bones  essentially  the  same  structural  features  are  present,  but  the  lamel- 
lar arrangement  is  much  less  regular. 

Although,  for  the  most  part,  the  intercellular  substance  of  bone  is, 
by  the  ordinary  modes  of  preparation,  apparently  quite  destitute  of 
structure  beyond  that  indicated  by  its  lamellation,  we  yet  find  in  cer- 
tain portions  a  well-defined  system  of  fibres.  If,  in  a  decalcified  bone, 
some  of  the  external  lamellae  are  torn  off,  numerous  fine,  fibriliated, 
spicula-like  projections  are  seen  hanging  on  to  the  inner  surface  of  the 
separated  fragments.  These  are  the  so-called  Sharpey's  fibres,  which, 
passing  inward  from  the  periosteum,  pierce  the  bone  either  obliquely 
or  at  right  angles.  As  we  shall  see  when  studying  the  development  of 
bone,  these  Sharpey's  fibres  are  the  remains  of  fibriliated  connective- 
tissue  bundles,  which  originally  occupied  the  situation  now  filled  by 
bone.  Recent  investigations,  moreover,  have  led  to  the  belief  that  in 
bone  as  in  hyaline  cartilage  the  basement  substance  is  everywhere  deli- 
cately fibriliated,  but  we  have  not  time  in  these  lessons  to  consider 
the  methods  by  which  this  is  believed  to  be  demonstrable. 

2.  Periosteum. — The  periosteum  consists  chiefly  of  fibriliated  con- 
nective tissue,  with  a  few  elastic  fibres,  and  we  recognize  in  it  two 
layers  :  an  outer  layer,  composed  chiefly  of  firm,  dense  connective 
tissue,  which  is  continuous  with  the  muscular  aponeuroses  surrounding 
the  bone  ;  and  an  inner  layer,  which  is  looser  in  texture,  more  vascular, 
and  abundantly  furnished  with  variously  shaped  cells.  The  periosteum 
is  attached  to  the  bone  by  connective-tissue  fibres,  which  pass  from  the 
former  into  the  substance  of  the  latter,  the  attachment  being  firmer  at 
some  points  than  at  others,  as,  for  example,  near  the  extremities  of  the 
long  bones  and  at  the  points  of  insertion  of  the  tendons  and  ligaments. 

Blood-vessels  pass  also  from  the  periosteum  into  the  bone. 

3.  Marrow. — Marrow-tissue  is  found  in  the  central  or  medullary 
cavity  of  bones,  in  the  tiny  chambers  of  spongy  bone,  and  in  the  Haver- 
sian canals.  Sometimes  it  has  a  yellowish  color  and  is  fatty,  sometimes 
it  presents  itself  in  the  form  of  a  reddish  pulp.  Red  marrow  is  found  in 
embryos  and  in  young  animals,  and  in  adults  in  certain  small  bones  and 
the  vertebrae. 

In  certain  animals,  such  as  the  rabbit  and  guinea-pig,  red  marrow  is 
found  in  most  of  the  bones,  even  in  adult  life.  In  adult  man,  under  nor- 
mal conditions,  the  marrow,  except  in  the  vertebrae,  ribs,  and  certain 
small  bones,  is  yellow.  Yellow  marrow  differs  from  the  red  in  that  it 
contains  a  large  amount  of  fat,  sometimes  consisting  almost  exclusively 
of  fat-cells. 

We  find  in  red  marrow,  which  is  best  adapted  for  study,  blood-vessels 
and  spindle-shaped  or  branching  cells,  which  constitute  the  supporting 
framework  of  the  tissue.  In  the  interstices  of  the  latter  lie  several  dis- 
tinct kinds  of  cells  :  1.  Fat-cells  ;  2.  smaller  and  larger  spheroidal  cells, 
having  essentially  the  same  structure  and  character  as  the  lymph-cells, 
and  called,  par  excellence,  marrow-cells  ;  3.  cells  somewhat  larger  than 
the  last  mentioned,  with,  usually,  a  single  very  irregular-shaped  and 
sharply  defined  nucleus  ;  4.  very  large  granular  cells,  which  usually  have 
several  nuclei  scattered  through  the  cell-body,  or  grouped  on  one  side  the 
so-called  myeloplaxes  or  giant  cells.     It  is  not  improbable  that  the  two 


40  NORMAL   HISTOLOGY. 

last  varieties  are  only  modified  forms  of  the  same  kind  of  cells.  In  the 
marrow  of  developing  bone  are  seen  spheroidal,  or  irregularly  cuboidal, 
large  granular  cells,  with  commonly  oval  nuclei,  usually  situated  at  one 
side  of  the  cell-body.  These  are  the  so-called  osteoblasts,  with  which 
we  shall  become  better  acquainted  when  we  study  the  process  of  bone- 
development. 

In  addition  to  the  above  cell-forms,  red  blood-cells,  escaped  from 
the  blood-vessels,  are  usually  seen  in  abundance.  Not  infrequently, 
when  fresh  marrow  is  studied,  cells  are  seen  which  in  many  respects  are 
like  the  true  marrow-cells,  but  which,  with  a  distinct  nucleus,  have  a 
homogeneous  cell-body  resembling  in  its  color  the  red  blood-cells. 
These  cells  are  the  so-called  nucleated red  blood-cells,  and  are  believed 
by  some  observers  to  be  destined  to  lose  their  nuclei  in  some  way 
or  other,  and  assume  the  character  of  the  ordinary  red  blood-cells. 
Those  who  advocate  this  view  regard  the  marrow  of  bones  as  one  of 
the  blood-producing  tissues  of  the  body.  The  transformation  of  these 
cells  into  red  blood-cells  has  never  been  directly  observed,  and  as  the 
peculiar  appearance  which  they  present  can  be  accounted  for  on 
other  grounds,  the  formation  of  red  blood-cells  in  the  marrow,  while 
not  improbable,  cannot  yet  be  regarded  as  definitely  demonstrated. 

PRACTICAL    STUDY. 

Decalcified  Bone. — To  obtain  a  general  view  of  the  structure  of 
bone  we  have  recourse  to  transverse  and  longitudinal  sections  of  one 
of  the  long  bones  (from  man  or  the  lower  animals,  such  as  the  rabbit 
or  dog),  which  has  been  freed  from  its  lime  salts  by  soaking  in  dilute 
acids  and  rendered  so  soft  as  to  be  readily  cut  with  a  razor. 

Although  various  acids,  such  as  nitric  and  hydrochloric,  effect  the 
decalcification  of  bone,  solutions  of  chromic  or  picric  acids  are  prefera- 
ble, because,  while  very  perfectly  removing  the  lime  salts,  they  harden 
and  preserve  the  soft  structures  in  a  most  satisfactory  manner.  As 
the  salts  of  lime,  as  they  exist  in  bone,  do  not  undergo  rapid  solution 
in  these  acids,  the  bits  of  bone  which  are  to  be  decalcified  should  be 
small,  or  the  process  will  be  a  very  protracted  one.  They  should  not 
at  most  be  larger  than  a  cubic  centimetre,  and  if  they  are  half  that 
size  the  process  will  be  much  more  rapidly  completed.  The  quantity 
of  fluid  should  also  be  quite  large  (200  to  300  cubic  centimetres  to  a 
bit  of  bone  of  the  above  size).  Picric  acid  is  on  the  whole  to  be  pre- 
ferred, because  the  chromic  acid  often  leaves  the  tissues  in  a  granular 
or  cloudy  condition,  which  interferes  with  subsequent  study.  If  chromic 
acid  be  employed,  the  bit  of  bone  is  put  first  into  a  weak  aqueous  solu- 
tion (1  in  600)  ;  in  a  couple  of  days  it  is  transferred  to  a  fresh  solution 
(1  in  400),  and  again  in  a  couple  of  days  to  another  solution  of  1  in 
200.  A  stronger  solution  than  this  should  never  be  used,  but  this 
should  be  renewed  every  few  days  and  the  bottle  frequently  shaken. 
In  two  or  three  weeks  the  process  will  probably  be  completed ;  this 
can  be  ascertained  by  passing  a  fine  needle  into  the  preparation.  If 
it  be  desirable  to  hasten  the  process,  after  a  week  or  ten  days'  soaking 
in  chromic  acid,  as  directed,  a  little  nitric  acid  may  be  added  to  the 


CARTILAGE— BONE — TEETH.  4 1 

solution  (1  c.c.  to  100  ex.).  The  previous  action  of  the  chromic  acid 
will  prevent  the  swelling  and  partial  destruction  of  the  soft  parts, 
which  nitric  acid  alone  causes.  If  picric  acid  be  employed,  a  saturated 
aqueous  solution  should  be  used,  the  preparation  frequently  shaken, 
and  additional  crystals  of  the  acid  occasionally  added. 

When  the  bone  has  become  sufficiently  soft  by  either  of  these 
methods,  it  is  allowed  to  soak  for  a  day  in  water  to  remove  the  excess 
of  acid,  and  then  hardened  and  preserved  in  alcohol.  The  picric  acid 
preparation  will  have  to  soak  a  day  or  two  in  alcohol  before  the  acid 
will  be  entirely  removed.  Sections  are  made  transversely  and  longi- 
tudinally by  the  usual  methods,  the  preparation  being  embedded  in 
wax  or  a  bit  of  hardened  liver,  if  too  small  to  hold  in  the  fingers. 

Bone  decalcified  in  chromic  acid  is  best  stained  in  hematoxylin; 
that  softened  by  picric  acid  shows  the  structure  very  beautifully  after 
staining  with  picro-carmine.  The  specimens  may  be  studied  in  gly- 
cerine, and  may  be  preserved  in  this  or  in  Canada  balsam.  If  the 
periosteum  has  not  been  removed,  its  structure  and  relation  to  the 
bone  is  well  shown. 

Sections  of  Hard  Bone. — In  such  preparations  as  the  above  which 
are  mounted  in  fluids  the  canaliculi  are  for  the  most  part  invisible  be- 
cause the  fluids  which  fill  them  possess  very  nearly  the  same  refractive 
power  as  the  basement  substance.  The  cell-spaces  and  canaliculi  are 
best  studied  in  thin  sections  of  hard  bone  which  have  been  macerated 
for  some  time,  so  as  to  remove  the  medullary  fat  and  other  soft  parts, 
and  then  dried.  Transverse  or  longitudinal  sections  may  be  made,  the 
latter  being  most  easily  prepared  because  sections  in  this  direction 
are  less  brittle.  A  small  piece,  as  thin  as  possible,  should  be  sawn 
from  the  bone  (a  diaphysis  of  a  human  long  bone  answers  very  well) 
in  the  proper  direction  ;  this  is  ground  down  very  thin  on  a  whetstone 
or  grindstone,  or  on  a  plate  of  glass  with  emery  powder,  the  section 
being  held  down  with  the  ball  of  the  finger  or  a  bit  of  soft  cork.  When 
it  has  become  quite  thin,  so  as  to  be  almost  transparent,  it  is  polished 
on  a  dry  oil-stone  free  from  grease,  and  then  carefully  washed  and 
brushed  under  water  with  a  fine  pencil  to  remove  particles  of  dirt.  It 
is  now  allowed  to  dry,  and  may  be  mounted  dry  in  a  cell  made  by  a 
ring  of  asphalt  varnish  and  covered,  the  cover  being  cemented  down 
by  a  rim  of  the  same  material ;  or  it  may  be  mounted  in  Canada  bal- 
sam. In  the  latter  case  the  semi-fluid  Canada  balsam,  such  as  is  used 
for  ordinary  mounting,  should  not  be  employed,  because  it  would  run 
into  the  lacunae  and  canaliculi,  and  render  them  invisible.  A  bit  of 
quite  hard  and  solid  balsam  should  be  placed  on  a  slide  and  heated 
until  it  melts  ;  just  as  it  begins  to  fairly  cool,  but  before  it  gets  at  all 
hard,  the  slip  of  bone  is  quickly  immersed  in  the  drop  and  covered. 
If  the  proper  moment  has  been  chosen,  when  the  balsam  is  neither  too 
hot  nor  too  cold,  the  unevennesses  upon  the  surface  of  the  section, 
which  are  seen  when  it  is  mounted  dry,  are  entirely  concealed,  and 
the  lacunae  and  canaliculi  are  clearly  defined  by  reason  of  the  air  with 
which  they  are  filled.  Usually,  however,  in  the  most  successful  pre- 
parations, in  the  very  thin  parts  or  at  the  edges,  part  of  the  canaliculi 
have  become  filled  with  the  balsam  and  rendered  invisible. 


42  NORMAL   HISTOLOGY. 

Marrow. — A  long  bone,  from  a  rabbit  or  from  a  child,  should  be 
broken  across  and  a  little  of  the  red  marrow  scooped  out  and  laid  for 
twenty-four  hours  in  a  mixture  of  alcohol  and  water  (i  to  2).  Fragments 
are  then  teased  on  a  slide  in  equal  parts  of  glycerine  and  water  which  has 
been  colored  with  eosin.  When  the  bits  have  been  thoroughly  teased 
and  are  sufficiently  stained,  they  are  covered  and  studied.  By  this 
process  the  red  blood-cells  are  destroyed  and  the  characters  of  the 
so-called  nucleated  red  blood-cells  somewhat  changed.  For  the  study 
of  the  latter  fresh  marrow  should  be  teased  on  a  slide  in  the  indifferent 
salt  solution. 

Development  of  Bone. 

At  a  certain  period  of  embryonic  life  no  bone-tissue  is  found  in  the 
body,  the  parts  where  it  is  finally  to  be  being  occupied  either  by  car- 
tilage or  fibrillar  connective  tissue.  Out  of  these  tissues  the  bone  is 
developed  by  a  process  which,  though  presenting  considerable  differ- 
ences in  detail  in  various  parts  of  the  body,  is  yet,  in  its  essential 
nature  the  same  in  all.  We  recognize  three  ways  in  which  bone  is 
developed:  1.  In  the  substance  of  pre-existing  cartilage — infra-carti- 
laginous ;  2.  beneath  the  periosteum — subperiosteal;  3.  in  the  sub- 
stance of  pre-existing  fibrillar  connective-tissue  membranes — iiitra- 
membra?ious. 

In  all  of  these  modes  of  bone-formation  the  new  bone  seems  to  be 
deposited  under  the  influence  of  certain  large,  granular,  usually  sphe- 
roidal or  cubical  nucleated  cells,  called  osteoblasts. 

1.  When  bone  is  formed  from  cartilage,  the  latter  bears  a  general 
resemblance,  in  shape,  to  the  finished  bone.  The  first  change  which 
we  notice  in  such  a  cartilage  which  is  about  to  undergo  ossification, 
is  that  at  a  certain  point — if  it  be  a  long  bone,  at  about  the  middle  of 
the  diaphysis — the  cartilage-cells  begin  to  enlarge,  the  basement  sub- 
stance between  them  becoming  partially  absorbed,  and  what  remains 
of  the  latter  becoming  infiltrated  with  fine  granules  of  lime  salts. 

Around  this  calcified  portion  we  find  the  blood-vessels  of  the  peri- 
chondrium accompanied  by  marrow-tissue  and  the  above-mentioned 
osteoblasts,  growing  into  the  calcified  cartilage,  absorbing  the  latter 
as  they  go,  and  forming  irregular  channels  or  cavities  called  medullary 
spaces.  These  channels  are  at  first  separated  from  one  another  by 
narrow,  irregular  septa  of  the  cartilage  basement  substance  which  re- 
mains unabsorbed,  and  are  lined  by  layers  of  osteoblasts  by  whose 
agency  the  septa  become  covered  with  new  bone  in  a  manner  pres- 
ently to  be  described. 

The  region  in  which  this  new  bone  is  first  deposited  is  called  the 
ossification  zone.  As  the  blood-vessels,  accompanied  by  the  osteo- 
blasts and  marrow-tissue,  proceed  farther  and  farther  into  the  cartilage, 
channelling  out  the  medullary  spaces  as  they  go,  we  always  find,  just 
in  advance  of  the  ends  of  the  blood-vessels  and  the  extremity  of  the 
spaces  in  which  they  lie,  a  zone  of  calcified  cartilage  ;  and  beyond  this 
cartilage  tissue  which  apparently  prepares  the  way  for  the  advancing 
marrow-spaces  and  newly  forming  bone  by  very  characteristic  modifi- 
cations, chiefly  in  the  form  and  arrangements  of  its  ceils. 


CARTILAGE — BONE — TEETH.  43 

If  we  examine  the  cartilage  at  a  considerable  distance  from  the 
line  of  ossification  we  find  the  ordinary  appearance  of  hyaline  cartilage 
with  more  or  less  flattened  cells.  Approaching  now  the  zone  of  ossi- 
fication, we  find  that  the  cells  are  larger,  are  arranged  in  rows  or 
groups  of  frequently  4,  8,  or  16,  etc.,  the  intercellular  substance  being 
less  in  amount  corresponding  to  the  increase  in  size  and  number  of 
the  cells.  Farther  inward  we  find  the  cells  still  more  plainly  arranged 
in  rows,  very  large,  sometimes  globular  or  flattened  against  one 
another,  and  the  basement  substance  reduced  to  quite  thin  septa,  en- 
closing spaces  in  which  the  rows  of  large  cartilage-cells  lie.  Then 
comes  a  narrow  zone,  in  which  the  septa  of  the  basement  substance 
are  filled  with  fine  granules  of  lime  salts — calcification  zone.  Here 
the  cartilage- cells  have  assumed  a  peculiar  granular  character.  Fi- 
nally, we  find  that  the  lime  salts  have  disappeared  from  the  septa,  and 
that  the  spaces  which  contained  the  large  granular  cartilage-cells  have 
become  continuous  with  the  advancing,  vascular,  bone-walled,  narrow 
cavities  above  described.  It  is  to  be  distinctly  understood  that  the 
calcification  zone  is  not  bone,  but  only  calcified  cartilage ;  the  true 
bone  being  first  formed  after  this  lime  has  disappeared,  on  the  surface 
of  the  septa  in  which  it  was  temporarily  deposited — for  what  purpose, 
we  do  not  know.  Turning  our  attention  now  to  the  exact  way  in 
which  the  bone  is  formed  under  the  influence  of  the  osteoblasts,  we 
find  that,  just  beneath  these  cells  as  they  lie  along  the  walls  of  the  new- 
formed  medullary  spaces,  the  basement  substance  of  true  bone  begins 
to  be  deposited,  at  first  in  the  form  of  a  narrow  shell  beneath  each 
osteoblast.  These  deposits,  which  on  cross-sections  have  a  crescentic 
shape,  become  thicker  and  thicker,  rising  up  around  the  cell,  which 
they  finally  enclose — the  enclosed  osteoblast  becoming,  as  it  would 
seem,  a  bone-cell.  This  process  going  on  around  each  osteoblast,  the 
walls  of  the  medullary  cavities  soon  become  covered  with  a  layer  of 
bone  containing  bone-cells.  New  osteoblasts  appear  on  the  walls, 
and  in  turn  become  enclosed  in  a  layer  of  bone,  and  thus  the  lamellar 
arrangement  of  bone-tissue  is  produced.  The  remains  of  cartilage 
thus  covered  by  bone,  in  the  septa,  finally  disappear  in  a  manner  un- 
known to  us. 

2.  Hand  in  hand  with  the  formation  of  bone  within  the  cartilage, 
new  bone  is  formed  on  its  surface  beneath  the  perichondrium,  which 
thus  becomes  periosteum.  The  process  of  sub-periosteal  ossification, 
by  which  the  bone  increases  in  thickness,  is  dependent  also  upon  the 
presence  of  osteoblasts.  We  find  these  arranging  themselves  along 
the  blood-vessels  which  enter  the  bone,  and  along  the  inner  layer  of 
connective-tissue  fibres  of  the  periosteum,  and  bone  is  formed  around 
them  in  the  manner  above  described.  New  bone  thus  formed  at  the 
surface  appears  at  first  by  no  means  in  the  form  of  smooth,  contin- 
uous layers,  but  as  the  blood-vessels  and  connective-tissue  bundles, 
along  which  the  osteoblasts  lie,  are  arranged  at  varying  angles  with 
the  surface  of  the  bone  and  with  each  other,  the  effect  is  to  produce 
irregular  branching  cavities,  upon  whose  walls  the  new  layers  of  bone 
are  deposited.  When  these  branching  cavities  become  filled,  with  the 
exception  of  the  space  occupied  by  the  blood-vessels  and  marrow- 


44  NORMAL   HISTOLOGY. 

tissue,  by  successive  lamellae  of  bone,  they  constitute  the  structures 
with  which  we  are  already  familiar,  under  the  name  of  Haversian 
canals  and  Haversian  lamellae.  Where  the  formation  of  bone  has 
taken  place  along  bundles  of  connective  tissue,  these  bundles  some- 
times persist  for  a  long  time,  in  a  modified  form,  between  the  sets  of 
lamellae,  and  constitute  the  above-mentioned  Sharpey's  fibres.  Thus, 
by  the  transformation  of  cartilage  and  apposition  at  the  surface,  the 
long  bones  are  formed.  In  these  bones  the  ossification  progresses 
toward  the  epiphyses,  where  independent  centres  of  ossification  are  es- 
tablished. The  lines  of  ossification  approach  each  other,  and  finally, 
when  the  process  of  growth  in  the  bone  is  complete,  the  band  of  carti- 
lage which  separated  them  disappears,  and  epiphysis  and  diaphysis 
join  to  form  a  single  bone.  As  the  bone  grows  by  apposition  beneath 
the  periosteum,  the  osseous  tissue  which  was  first  formed  in  the  dia- 
physis is  absorbed,  and  the  medullary  cavity  is  formed  in  the  place 
which  it  originally  occupied. 

3.  When  bone  is  formed  in  membranes  of  fibrillar  connective  tissue, 
as  in  the  skull-cap,  we  notice,  first,  that  some  of  the  interlacing  bun- 
dles which  occupy  the  place  of  the  future  bone  become  infiltrated 
with  lime  salts ;  along  these  calcareous  bundles  cells  become  very 
abundant,  osteoblasts  appear  and  arrange  themselves,  and  bone  forms 
around  them  just  as  in  the  other  varieties  of  bone-formation.  Blood- 
vessels and  marrow-tissue  lie  between  the  new-formed  layers  of  bone, 
so  that  at  a  certain  period  of  embryonic  life  the  bones  of  the  skull-cap 
consist  of  a  series  of  bony  lamellae,  arranged  so  as  to  enclose  branch- 
ing and  communicating  cavities,  which  are  occupied  by  blood-vessels 
and  marrow-tissue,  and  whose  walls  are  lined  with  osteoblasts.  A 
well-defined  periosteum  is  finally  formed,  beneath  which  successive 
layers  of  new  osseous  tissue  are  deposited,  and  thus  the  bone  increases 
in  thickness  and  acquires  its  smooth  surfaces. 

The  mode  of  origin  of  the  osteoblasts  is  still  very  obscure.  Many 
investigators  believe  that  in  the  intra-cartilaginous  ossification  they  are 
the  large  cartilage-cells  which  we  see  at  the  calcification  line,  which  in 
contact  with  the  blood-vessels  become  so  modified  in  form  and  func- 
tion as  to  assume  the  role  of  bone-formers.  Others  assert  that  the 
large  cartilage-cells  disintegrate  and  disappear,  and  that  the  osteo- 
blasts are  produced  from  cells  which  accompany  the  blood-vessels. 
Still  others  regard  them  as  white  blood-cells,  modified  and  endowed 
with  new  functional  powers.  In  the  intra-membranous  and  subperi- 
osteal ossification,  many  suppose  that  they  are  formed  from  connective- 
tissue  cells.  So  little  is  absolutely  known,  however,  as  to  their  genesis, 
that  while  recognizing  their  importance  in  bone-formation,  we  can 
regard  none  of  these  various  theories  as  to  their  origin  as  definitely 
established. 

PRACTICAL   STUDY. 

Intra-cartilaginous  and  Subperiosteal  Ossification. — A  long  bone, 
from  a  nearly  mature  foetus  or  a  young  animal,  should  be  carefully 
removed  without  injuring  the  periosteum,  and  decalcified  with  picric 


CARTILAGE— BONE— TEETH.  45 

or  chromic  acid.  Thin  longitudinal  sections  are  made  with  a  razor 
through  the  ossification  zone,  embracing  the  tissue  for  a  considerable 
distance  on  either  side  of  it.  The  sections  are  stained,  first  with 
hematoxylin  and  then  with  eosin,  and  mounted  in  glycerine  or  Canada 
balsam. 

The  basement  substance  of  the  cartilage  and  the  cell-nuclei  will  be 
stained  violet,  while  the  cell-bodies  and  the  new-formed  bone  are 
stained  rose-red. 

Intra-membranous  Ossification. — To  study  the  early  stages  of  this 
process,  a  young  embryo  (if  from  the  sheep  or  pig,  4  to  6  cms.  long) 
should  be  soaked  for  a  few  days  in  Miiller's  fluid,  and  a  bit  corre- 
sponding to  the  portion  of  one  of  the  parietal  bones  cut  out,  and  the 
skin,  muscles,  and  dura  mater  torn  away  with  forceps  under  water. 
The  membrane  in  which  ossification  is  occurring  is  now  to  be  care- 
fully brushed  with  a  stiff  pencil  until  it  is  thin  enough  to  be  examined 
with  tolerably  high  powers.  It  is  stained  with  hematoxylin  and  eosin, 
and  mounted  in  glycerine  or  balsam. 

The  irregular  chambers,  lined  with  osteoblasts  and  filled  with 
blood-vessels  and  marrow,  which  are  formed  in  the  bones  of  the  skull- 
cap at  a  later  period,  are  well-shown  by  transverse  sections  through 
the  decalcified  skull-bones  of  an  older  foetus  (human,  at  about  6  or  7 
months,  or  from  the  beef  or  sheep,  16  to  20  cms.  long).  These 
should  be  stained  as  above,  and  mounted  in  Canada  balsam. 

Teeth. 

The  teeth  have  many  structural  features  in  common  with  bone. 
The  chief  bulk  of  the  tooth  is  made  up  of  a  homogeneous,  brittle 
basement  substance  much  harder  than  bone,  called  dentine.  The  den- 
tine contains  lime  salts,  and  is  permeated  by  a  multitude  of  fine 
branching  channels  which  radiate  from  a  central  cavity,  called  the 
pulp  cavity,  which  the  dentine  encloses. 

These  delicate  channels  in  the  dentine  are  analogous  with  the 
canaliculi  of  bone.  The  pulp  cavity  is  filled  with  a  soft  vascular 
tissue,  called  pulp,  containing  irregular-shaped,  often  branching  cells, 
and  nerves. 

Along  the  sides  of  the  pulp  cavity  lie  spheroidal  or  ovoid  cells, 
which  send  off  branches  into  the  pulp  and  also  into  the  above  men- 
tioned delicate  channels  in  the  dentine.  These  cells  are  called  odonto- 
blasts, or  de?itine  cells,  and  are  usually  considered  to  be  the  analogues 
of  the  bone-cells.  The  pulp  cavity  is  open  at  the  root  of  the  tooth 
for  the  admission  of  vessels  and  nerves.  Surrounding  the  root  of  the 
tooth  is  a  thin  layer  of  bone,  called  cement.  At  the  crown  of  the 
tooth  the  dentine  is  completely  covered  by  a  layer,  of  varying  thick- 
ness, of  an  extremely  hard  substance  called  enamel.  The  enamel 
consists  of  a  series  of  closely  packed,  small,  wavy  or  undulating 
prisms,  placed  edgewise  upon  the  surface  of  the  dentine,  and  covered, 
over  the  free  surface  of  the  tooth,  by  a  hard,  tough,  structureless  mem- 
brane, called  the  enamel  cuticle. 


46  NORMAL   HISTOLOGY. 


PRACTICAL    STUDY. 

Hard  Teeth. — To  study  the  hard  parts  of  teeth,  thin  sections  of 
a  macerated  and  dried  tooth  should  be  ground  down '  by  the  method 
described  when  we  were  studying  hard  bone,  and  mounted,  with  the 
same  precautions,  in  hard  Canada  balsam. 

Decalcified  Teeth. — The  soft  parts  of  teeth  may  be  studied  in  sec- 
tions from  teeth  which  have  been  decalcified  with  picric  acid.  The 
tooth  should  be  broken  across,  so  as  to  expose  the  pulp  cavity  and 
hasten  the  action  of  the  solvent,  and  immersed  until  soft  in  a  saturated 
solution  cf  the  acid,  which  is  frequently  shaken,  and  to  which  fresh 
crystals  are  added  at  short  intervals.  Sections  are  stained  with  hsema- 
toxylin  and  eosin. 


CHAPTER   V. 

BLOOD  AND    LYMPH. 

Blood. 

Although  strikingly  different  in  some  of  their  physical  characters 
from  most  of  the  animal  tissues,  we  must  yet  regard  blood  and  lymph 
as  true  tissues,  in  which,  however,  unlike  most  others,  the  intercellular 
substance  is  fluid.  Let  us  first  consider  the  blood.  In  normal  human 
blood  we  find  suspended  in  a  colorless  fluid — the  plasma — three  dis- 
tinct classes  of  formed  elements  :  r.  colorless  blood-cells ;  2.  red 
blood-cells  ;  3.  variously  shaped  free  granules. 

t.  Colorless  Blood-cells,  or  Leucocytes. — These  are  small  spheroidal 
cells,  without  a  distinct  membrane,  the  cell-body  being  sometimes  al- 
most transparent,  or  very  finely  granular,  in  other  cases  quite  coarsely 
granular. 

They  possess  one  or  more  nuclei,  not  usually  visible  in  the  living 
cells,  and  of  varying  form — sometimes  spherical  or  dumb-bell  shaped, 
sometimes  having  the  form  of  a  bent  or  twisted  cylinder,  and  again 
entirely  irregular. 

These  cells  possess  the  power,  under  favorable  conditions,  of 
spontaneous  movement.  They  change  their  form  and  place.  While, 
when  in  a  state  of  rest,  they  assume  in  general  the  spheroidal  form,  as 
above  stated,  we  find  that  when  they  become  active  they  send  out 
variously  shaped  processes,  some  fine  and  delicate,  others  broad  and 
of  very  irregular  shape.  We  often  see,  after  a  process  has  been 
thrown  out,  that  it  becomes  gradually  larger  and  larger,  the  cell-body 
becoming  correspondingly  smaller,  until  finally  the  whole  cell  seems 
to  have  passed  over  into  the  process,  thus  moving  forward. 

Sometimes  processes  are  thrown  out  and  again  withdrawn,  and  not 
infrequently  the  whole  cell  flattens  out  into  an  irregular-shaped  mass, 
so  thin  as  to  be  almost  invisible.  Not  infrequently  clear  rounded 
spaces,  called  vacuoles,  suddenly  appear  in  the  cell-body  during  its 
movements,  and  either  remain  for  some  time,  or  soon  disappear  as 
suddenly  as  they  came. 

These  movements  are  called  amoeboid  movements  ;  they  are  always 
very  slow,  and  are  greatly  influenced  by  the  temperature,  density, 
and  oxygen-content  of  the  fluid  in  which  they  lie.  By  virtue  of  this 
locomotive  power  the  white  blood-cells  perform  certain  evolutions 
within  the  vessels  ;  they  escape  through  their  walls,  and,  sometimes 
singly,  sometimes  in  vast  numbers,  move  through  the  tissues  in  the 
larger  and  smaller  lymph-spaces.     This  emigration  of  white  blood- 


43  NORMAL   HISTOLOGY. 

cells  occurs,  apparently,  to  a  slight  extent,  under  normal  conditions  ; 
but  it  is  under  pathological  conditions  that  it  is  most  active.  The  nu- 
clei of  these  cells  become  visible  on  the  application  of  a  variety  of 
agents  which  determine  the  death  of  the  cells,  such  as  acetic  acid, 
dilute  alcohol,  and  certain  coloring  agents.  The  cells  vary  greatly  in 
size,  but  on  the  average  are,  in  man,  larger  than  the  red  cells. 

2.  Red  Blood-cells. — These  cells  having  in  man  the  form  of  bi- 
concave disks,  consist  apparently  simply  of  a  cell-body  without  mem- 
brane or  nucleus — at  least  in  the  adult  human  blood-cells  no  such 
structure  is  demonstrable  by  the  ordinary  modes  of  preparation. 

Although  when  crowded  together  in  great  numbers  the  blood-cells 
present  a  red  color,  when  seen  separately  they  have  a  greenish  yellow 
tint. 

The  cell-body  is  very  soft  and  pliable,  jelly-like,  changing  its  shape 
on  the  slightest  pressure.  It  is  more  deeply  colored  at  the  periphery, 
when  seen  from  the  side,  because  of  its  greater  thickness  at  that  part. 
Owing  to  the  peculiar  shape  of  the  cell,  it  acts  as  a  lens  upon  the 
light  passing  through  it,  and  its  central  portion  is  either  light  or  dark, 
depending  upon  whether  the  objective  is  approached  to  or  withdrawn 
from  it.  When  examined  fresh  in  the  plasma,  many  of  the  cells  ar- 
range themselves  closely  together  side  by  side,  in  longer  and  shorter 
rows.  On  being  drawn  from  the  vessels,  if  the  plasma  be  allowed  to 
evaporate,  or  if  certain  fluids,  such  as  strong  solutions  of  common 
salt  or  bichromate  of  potassium,  be  added,  a  part  of  the  red  blood- 
cells  lose  their  regular  shape,  their  edges  become  crenulate  and 
jagged,  and  they  sometimes  seem  to  become  smaller;  finally,  they 
assume  the  form  of  irregular  globular  masses,  beset  with  short,  blunt 
spines.  Various  other  irregular  forms  are  produced  under  the  same 
circumstances,  which  it  is  not  necessary  to  describe  here.  The  addi- 
tion of  water  causes  the  spines  to  disappear,  and  the  cells  swell  up 
again  and  assume  a  globular  form.  The  coloring  matter  is  dissolved 
by  the  water,  which  itself  becomes  colored. 

These  changes  in  form,  produced  by  certain  chemical  agents  and 
by  changes  in  the  density  of  the  fluid  in  which  the  cells  lie,  should  not 
be  mistaken  for  the  expression  of  vitality,  or  regarded  as  analogous 
with  the  amoeboid  movements  of  the  white  blood-cells.  Not  infre- 
quently in  normal  blood,  very  often  under  pathological  conditions, 
red.  blood-cells  are  seen  which  are  much  smaller  than  the  above  de- 
scribed forms,  and  are  spheroidal  in  shape.  The  red  blood-cells  are 
much  more  abundant  than  the  white,  there  being  in  normal  human 
blood  about  350  to  500  of  the  former  to  one  of  the  latter.  The  red 
blood-cells  owe  their  color  as  well  as  their  power  of  performing  certain 
important  physiological  functions  to  the  presence  in  them  of  a  crys- 
tallizable  substance  called  haemoglobine. 

The  shape  of  haemoglobine  crystals  obtained  from  the  blood  of 
different  animals  is  not  always  the  same  ;  those  from  the  human  blood 
have  in  general  the  form  of  rhombic  prisms. 

The  free  granules,  which  are  always  found  in  greater  or  less  num- 
ber in  human  blood,  are  chiefly  of  two  kinds  :  small  spherical  bodies, 
somewhat  resembling  fat-droplets,  and  variously  shaped,  often  angular 


BLOOD   AND   LYMPH.  49 

masses,  looking  not  unlike  fragments  of  white  blood-cells.  The  na- 
ture of  these  granules  is  yet  undetermined. 

The  above  description  of  the  blood-cells  of  man  applies,  with  few 
exceptions,  to  other  mammalia.  In  other  vertebrates,  however,  we  find 
a  marked  difference  in  the  form  and  character,  particularly  of  the  red 
cells.  They  are,  for  the  most  part,  oval,  and  possess  a  distinct  nucleus, 
which  often  projects  from  both  sides  of  the  disk. 

The  intercellular  substance,  or  plasma  of  freshly  drawn  blood,  is 
perfectly  homogeneous ;  but  if  we  allow  it  to  coagulate  we  find,  on 
subjecting  it  to  microscopical  examination,  that  in  addition  to  the 
above  described  elements,  a  multitude  of  delicate  filaments  lie  among 
the  cells,  stretching  in  all  directions  and  joining  each  other  at  frequent 
intervals,  forming  an  irregular  meshed  net.  We  find,  moreover,  if  we 
examine  a  clot  from  which  the  red  cells  have  been  carefully  removed 
or  rendered  invisible,  that  in  many  places  the  filaments  are  grouped 
around  irregular-shaped  granules,  looking  like  those  above  described 
in  normal  blood  ;  and  if  the  process  of  coagulation  be  carefully  ob- 
served, they  may  be  seen  to  actually  shoot  out  from  these  granules, 
which  thus  seem  to  form  starting-points  for  their  formation.  The  sub- 
stance which  thus  separates  from  the  plasma  is  called  fibrine. 

Lymph. 

In  lymph  we  find  spherical  cells,  identical  in  structure  with  the 
white  blood-cells  and  variously  shaped  granules,  composed  apparently 
of  a  combination  of  albuminoid  material  with  fat.  These  granules,  in 
that  variety  of  lymph  called  chyle,  is  so  abundant  as  to  give  the  fluid 
a  milky  appearance. 

Direct  observation  has  shown  that,. in  some  animals  at  least,  the 
white  blood-cells  can  multiply  by  division.  Whether  the  cells  which 
supply  the  place  of  those  which  seem  to  be  used  up  in  the  process  of 
growth  and  reparation  are  produced  in  this  way,  and  if  so,  whether  the 
division  occurs  in  the  blood-  or  lymph-vessels,  or  in  the  cell-spaces  of 
the  connective  tissue,  or  in  certain  special  organs,  or  whether  they  are 
produced  in  a  manner  entirely  unknown  to  us — these  are  questions 
not  only  of  theoretical  but  of  practical  interest ;  but,  in  spite  of  much 
research  and  the  accumulation  of  many  observations  bearing  on  the 
matter,  we  are  still  unable  to  give  them  a  definite  answer.  Still  more 
obscure,  if  possible,  is  the  origin  of  the  red  blood-cells.  Although  in 
the  adult  man  they  seem  to  possess  no  nucleus,  yet  in  embryonic  life 
they  certainly  are  furnished  with  that  structure,  we  find  nucleated  red 
blood-cells.  Now,  it  has  been  recently  shown  that  in  certain  parts  of 
the  body,  in  adult  life,  cells  occur  which  in  many  respects  resemble 
the  nucleated  red  blood-cells  of  the  embryo  ;  such  cells  are  found,  for 
example,  in  the  spleen,  in  the  red  marrow  of  bones,  etc. 

The  most  plausible  theory  in  regard  to  the  matter  is  that  in  certain 
parts  of  the  body — spleen,  marrow,  lymph-glands,  and  liver — white 
blood-cells  are  produced,  a  part  of  which  are  changed  into  the  red 
blood-cells.  The  so-called  nucleated  red  blood-cells  are  supposed  to 
be  intermediate  forms.  It  must  be  remembered,  however,  that  this  is 
4 


50  NORMAL   HISTOLOGY. 

only  theory  as  yet,  and  many  observers  do  not  ascribe  to  the  so-called 
nucleated  red  blood-cells  the  significance  upon  which  the  advocates 
of  this  theory  insist. 

I    PRACTICAL    STUDY. 

Fresh  Human  Blood. — This  may  be  obtained  by  tying  a  cord  tightly 
around  the  finger,  to  cause  congestion,  and  then  pricking  it  sharply 
with  a  needle.  A  clean  cover-glass  and  slide  being  at  hand,  and  the 
microscope  being  in  readiness,  the  cover-glass  is  touched  to  the  drop  of 
blood  so  that  a  small  droplet  adheres.  The  cover  is  now  carefully  low- 
ered onto  the  slide,  no  pressure  being  exerted  beyond  its  own  weight. 
The  film  of  blood  beneath  the  cover  should  be  very  thin,  or  the  crowd- 
ing of  the  blood-cells  will  interfere  with  the  observation.  The  speci- 
men should  be  examined  at  once,  when  the  various  above  described 
elements  will  be  readily  recognized.  As  the  plasma  evaporates  from 
the  edges  the  changes  in  form  of  the  red  blood-cells  will  be  seen. 
Finally,  a  drop  of  water  should  be  added  at  the  edge  of  the  cover-glass 
and  its  effect  observed. 

White  Blood-cells. — In  order  to  study  more  thoroughly  the  form  of 
the  white  blood-cells,  and  the  various  shapes  which  their  nuclei  pre- 
sent, a  drop  of  blood  should  be  placed  on  a  slide  and  mixed  with  a 
few  drops  of  dilute  alcohol  (alcohol  i,  water  3).  After  a  few  moments 
a  drop  of  eosin  is  added  to  the  specimen,  and  it  is  covered  and  studied 
at  once. 

Amozboid  Movements. — These  are  most  conveniently  studied  in  the 
blood  or  lymph  from  one  of  the  cold-blooded  animals,  such  as  the 
frog,  for  they  occur  here  at  the  ordinary  temperature  of  the  air,  while 
artificial  heat  must  be  resorted  to  if  we  would  maintain  the  blood  of 
warm-blooded  animals  at  a  proper  temperature  for  the  observation  of 
these  movements.  A  shallow  cell — about  one  mm.  deep — having 
been  made  on  a  slide,  either  by  painting  a  narrow  ring  of  asphalt  varnish 
upon  it,  or  by  cementing  to  it  a  ring  made  for  the  purpose,  and  a  little 
sweet  oil  having  been  rubbed  over  the  rim  of  the  cell  so  as  to  form  an 
air-tight  chamber  of  the  latter  when  the  cover  is  put  on,  a  small  drop 
of  blood,  obtained  by  snipping  off  the  tip  of  one  of  the  carefully  cleansed 
toes,  is  put  on  the  cover-glass,  and  it  is  inverted  over  the  cell.  As  the 
amoeboid  movements  are  very  tardy  if  the  blood  evaporates  in  the  least, 
and  are  more  vigorous  if  the  blood  is  even  slightly  diluted,  it  is  well,  be- 
fore inverting  the  cover,  to  breathe  into  the  cell,  so  that  when  the  cover 
is  put  down  and  the  cavity  closed,  the  air  within,  in  contact  with  the 
blood,  may  be  charged  with  moisture. 

On  focussing  now  upon  the  specimen,  white  blood-cells  will  readily 
be  found  in  contact  with  the  cover-glass,  and  selecting  one  of  them 
which,  by  its  irregular  shape,  indicates  its  activity,  the  attention  must 
be  fixed  on  this  cell,  and  sketches  of  its  form  made  at  short  intervals. 

If  the  temperature  of  the  room  be  low  the  movements  may  be 
tardy  in  commencing,  and  they  can  be  hastened  by  holding  the  finger 
or  any  warm  object  for  a  moment  near  the  cover.  It  will  be  borne 
in  mind  that  in  the  frog's  blood  the  red  cells  are  oval  and  nucleated, 
and  that  they  are  also  larger  than  the  colorless  cells. 


BLOOD   AND    LYMPH.  5 1 

Fibrine. — A  small  quantity  of  blood  may  be  whipped  and  the  cells 
washed  from  the  clot  by  a  stream  of  water,  and  a  fragment  of  the 
remaining  substance  teased  in  water  on  a  slide  and  studied  ;  but  the 
objects  thus  obtained  are  not  altogether  satisfactory,  since  the  relation 
of  the  fibrillse  to  one  another  is  disturbed  and  no  light  is  thrown  on 
the  way  in  which  they  are  formed.  For  the  accomplishment  of  these 
ends,  the  following  method  may  be  employed  :  a  medium-sized  drop 
of  blood  is  received  on  a  slide  and  immediately  covered ;  after  a  few 
moments,  when  coagulation  has  occurred,  a  mixture  of  alcohol  and 
water,  i  to  2,  is  allowed  to  run  under  the  cover;  the  latter  may  be 
slightly  raised  at  the  edge  if  the  clotted  blood  prevents  the  fluid  from 
running  under;  this  decolorizes  the  red  blood-cells,  which  become 
nearly  invisible.  The  fibrine  may  now  be  seen  with  a  moderately  high 
magnifying  power,  but  it  can  be  brought  much  more  clearly  into  view 
if  a  dilute  solution  of  aniline  red  in  equal  parts  of  alcohol  and  water 
be  allowed  to  run  under  the  cover;  this  stains  the  white  blood-cells 
and  the  fibrine.  leaving  the  red  cells  uncolored.  The  object  of  adding 
the  dilute  alcohol  is  to  ged  rid  of  the  red  blood-cells,  whose  color,  when 
they  are  present  in  large  numbers,  almost  entirely  conceals  the  fibrine 
fibres. 


CHAPTER  VI. 

MUSCULAR   TISSUE. 

Certain  muscles  are  under  the  control  of  the  will,  and  are  called 
voluntary  muscles;  and  as  these  have,  as  we  shall  see  presently,  a 
very  characteristic  striation  of  their  structural  elements,  they  are  also 
called  striated  voluntary  muscles.  To  this  class  belong,  among  others, 
the  muscles  of  locomotion  and  the  voluntary  muscles  of  the  trunk 
and  head.  Other  muscles  are  not  under  control  of  the  will,  and  are 
hence  called  involuntary.  A  certain  portion  of  these  involuntary 
muscles  possess  the  same  striation  of  their  elements  as  the  voluntary 
muscles,  and  hence  are  called  involuntary  striated  muscles.  These 
are  found  in  the  heart.  In  another  kind  of  involuntary  muscle,  such 
as  is  found  in  the  intestine  and  bladder,  the  elements  do  not  possess 
the  same  kind  of  striations,  and  they  are  hence  called  smooth  or  non- 
striated  involuntary  muscles. 

Y\~e  have  thus  to  studv  three  kinds  of  muscles  : 

i.    Ir.voiuntarv  muscles  -1  f  sm-°?*  or  ""Striated. 

(  b.  striated. 
2.    Voluntary  muscles  (striated). 

i.  a. — Smooth   Muscular  Tissue. 

Smooth  muscular  tissue  is  made  up  of  very  much  elongated,  narrow, 
pointed,  usually  fusiform  nucleated  cells.       These  cells  are  commonly 

arranged  in  groups  or  bundles,  enclosed  in  connective  tissue  and  sup- 
plied with  blood-vessels  and  nerves.  The  cell-body,  although  usually 
fusiform,  is  sometimes  flattened  and  band-like,  often  divided  at  the 
ends  into  two  pointed  extremities.  Owing  to  pressure  from  adjacent 
parts,  the  fusiform  cell-bodies  are  often  more  or  less  flattened  at  the 
sides,  presenting  on  cross-section  an  irregular  polygonal  contour.  The 
cell-body  has  an  indistinct  longitudinal  striation.  is  otherwise  homo- 
geneous except  that  occasionally,  in  the  vicinity  of  the  nucleus,  a 
few  shining  granules  are  seen.  The  nucleus  is  narrow  and  much 
elongated,  rod-like,  pointed  or  rounded  at  the  ends,  and  commonly 
encloses  one  or  more  nucleoli.  It  usually  lies  near  the  middle  of  the 
cell,  which  is  often  thickened  or  bulging  at  that  point. 

These  ceils  lie  side  by  sice  or  lap  over  one  another  at  the  ends, 
and  are  joined  together  by  a  small  amount  of  an  albuminoid  cement 
substance,  which,  like  the  cementing  substance  of  endothelial  and 
other  cells,  is  stained  brown  or  black  after  exposure  to  light  by  dilute 


MUSCULAR   TISSUE.  53 

solutions  of  nitrate  of  silver.  These  smooth  muscle-cells  are  variously 
grouped  in  different  parts  of  the  body ;  sometimes  crowded  together 
in  solid  bundles,  which  are  surrounded  by  connective  tissue  and  ar- 
ranged in  layers  as  in  the  intestine  ;  sometimes  arranged  in  narrow 
interlacing  fascicles,  as  in  the  bladder ;  sometimes  scattered  singly 
through  certain  tissues  ;  and  again,  running  in  various  directions  and 
associated  with  bands  of  connective  tissue,  they  form  compact  masses, 
as  in  the  uterus. 

The  longitudinal  striation,  which,  under  favorable  circumstances,  is 
seen  on  the  cell-body,  is  not  a  mere  surface  marking,  but  extends  deep 
into  the  cell,  as  may  be  seen  in  transverse  sections  of  suitably  pre- 
pared cells,  where  fine  lines  are  observed  passing  inward  from  the 
periphery  of  the  cell  toward  the  nucleus.  The  blood-vessels  supply- 
ing this  tissue  form  for  the  most  part  elongated  net-works  throughout 
its  substance. 

PRACTICAL    STUDY. 

Isolated  Cells. — These  we  obtain  by  teasing  bits  of  the  tissue,  but 
as  they  are  firmly  bound  together  by  the  cementing  substance,  this 
must  first  be  dissolved  or  softened.  This  can  be  conveniently  accom- 
plished by  soaking  a  bit  of  the  tissue — the  wall  of  the  intestine,  for 
example — for  forty-eight  hours  in  an  aqueous  solution  of  bichromate 
of  potassium  (1  to  800 ).  Shreds  of  the  muscular  layer  are  torn  off, 
slightly  stained  with  hematoxylin,  washed,  and  then  carefully  teased 
in  glycerine  which  has  been  colored  by  eosin.  Since  the  cells  are 
apt  to  shrink  somewhat  under  the  action  of  the  bichromate,  it  is  well 
to  enclose  a  short  segment  of  the  intestine,  distended  with  the  solu- 
tion between  two  ligatures,  and  immerse  it  in  a  vessel  of  the  same 
solution. 

Transverse  and  Longitudinal  Sections. — The  intestine  of  the  cat 
is  -well  adapted  for  the  preparation  of  transverse  sections,  since  here 
the  muscle-ceils  are  unusually  large.  A  segment  having  been  dis- 
tended as  above  with  MuTiers  fluid,  it  should  be  immersed  for  ten 
days  in  the  same,  then  carefully  washed  and  put  for  a  day  or  two  in 
strong  alcohol.  A  bit  is  then  cut  out,  embedded  between  pieces  of 
hardened  liver,  and  thin  sections  made  in  a  direction  exactly  at  right 
angles  to  the  axis  of  the  gut.  The  sections  are  stamed  with  hema- 
toxylin and  mounted  in  glycerine.  In  such  a  preparation  two  layers 
of  muscle-cells  are  seen:  in  one  the  cells  are  seen  in  transverse,  in 
the  other  in  longitudinal  section.  Since  the  cells  lap  over  one  another 
in  the  transverse  sections,  the  forms  which  they  present  will  obviously 
differ,  depending  upon  whether  they  have  been  cut  across  at  die  level 
of  the  nucleus,  or  at  a  point  nearer  the  extremity.  In  such  a  prepa- 
ration, the  serosa  and  mucous  membrane  may  be  seen  on  opposite 
sides  of  the  muscular  layers. 

Treatment  with  Nitrate  of  Silver. — To  show  the  cementing  sub- 
stance between  the  cells,  a  portion  of  intestine — the  large  intestine  of 
the  frog  answers  well — should,  be  distended  with  air,  a  segment  en- 
closed between  two   ligatures,  and   the  surface  brushed  firmly  under 


54  NORMAL   HISTOLOGY. 

water  with  a  pencil,  to  remove  the  endothelium  which  covers  the  peri- 
toneal surface.  It  is  then  immersed  for  two  minutes  in  a  half  per  cent, 
solution  of  nitrate  of  silver,  washed  with  pure  water,  and  exposed  to 
the  light,  in  a  mixture  of  equal  parts  of  alcohol  and  water,  till  the  sur- 
face becomes  brown.  Small  bits  are  now  cut  out,  the  epithelium  from 
the  mucous  membrane  scraped  off,  and  the  object  mounted  in  glyce- 
rine with  the  external  surface  uppermost.  The  cells  will  be  seen  to 
be  bounded  by  brown  or  black  lines,  which  represent  the  cement  sub- 
stance. 

Muscle-cells  of  Frogs  Bladder. — Elegant  pictures  of  very  much 
elongated  slender  muscle-cells,  lying  singly  or  arranged  in  narrow  in- 
terlacing fascicles,  may  be  obtained  from  the  frog's  bladder  by  the  fol- 
lowing method  :  the  spinal  cord  of  a  frog  being  broken  up,  the  ab- 
dominal cavity  is  largely  opened  by  a  crucial  incision,  and  a  curved 
canula,  attached  to  a  small  syringe  filled  with  a  saturated  solution  of 
bichromate  of  potassium,  is  passed  into  the  cloaca  and  directed  for- 
ward into  the  bladder.  The  fluid  is  now  slowly  injected,  and  when 
the  bladder  is  partially  distended,  a  ligature  is  thrown  around  its  base, 
and  the  injection  continued  till  the  organ  is  fully  distended.  The 
ligature  is  now  drawn  tight  and  the  canula  withdrawn.  The  bladder 
is  cut  out,  still  distended,  and  put  in  the  same  bichromate  solution, 
where  it  remains  for  three  days,  when  it  is  washed  and  transferred  to 
alcohol.  After  twenty-four  hours  it  may  be  opened,  a  bit  cut  out,  the 
epithelium  carefully  brushed  from  the  inner  surface,  and  stained  with 
haematoxylin  and  eosin,  and  mounted  in  glycerine.  In  addition  to 
the  muscle-cells,  the  nuclei  of  the  endothelial  cells  covering  the  perito- 
neal surface  will  be  seen,  as  well  as  connective-tissue  cells  and  fibres. 

2.  Striated  Voluntary  Muscular  Tissue. 

As  the  involuntary  striated  or  heart  muscle  occupies  in  structure 
an  intermediate  position  between  the  smooth  and  the  voluntary  stri- 
ated muscle,  we  shall  find  it  advantageous  to  postpone  its  study  until 
we  have  considered  the  other  varieties.  Voluntary  striated  muscle,  to 
which  the  greater  part  of  the  muscular  tissue  of  the  body  belongs,  is 
made  up  of  narrow,  cylindrical,  cord-like  elements,  of  varying  length 
and  thickness,  called  muscular  fibres.  These  are  grouped  in  variously 
shaped  bundles  or  fascicles,  surrounded  by  connective-tissue  enve- 
lopes or  sheaths,  and  abundantly  supplied  with  blood-vessels  and 
nerves.  Let  us  first  study  the  structure  of  the  individual  fibres.  They 
consist  of  three  distinct  elements  :  i,  contractile  substance,  forming 
the  centre  and  making  up  most  of  the  bulk  of  the  fibre  ;  2,  nuclei, 
which  in  man  and  most  warm-blooded  animals  lie  scattered  upon  the 
surface  of  the  contractile  substance  ;  3,  the  sarcolenvma,  a  thin  homo- 
geneous sheath  or  tube,  which  tightly  encloses  the  other  elements. 
1.  If  we  examine  a  fresh  muscle-fibre,  or  one  which  has  been  hard- 
ened under  favorable  conditions  with  moderately  high  powers,  we  see 
that  the  contractile  substance  is  indistinctly  longitudinally  striated  ; 
and  if  we  treat  muscle  with  certain  chemical  agents,  such  as  chromic 
acid  or  its  salts,  we  find  that  by  slightly  teasing  the  fibres,  break  up 


MUSCULAR  TISSUE.  55 

along  the  longitudinal  striae  into  a  multitude  of  fine  fibrillar,  which 
are  called  primitive  muscle-ftbrillce.  Again,  if  we  examine  the  fresh 
or  hardened  fibres  still  further,  we  find  that  in  addition  to  the  longitu- 
dinal striations,  they  are  crossed  by  more  prominent,  narrow,  alter- 
nating, dark  and  light  bands  or  stripes,  the  relative  width  of  the  stripes 
varying  according  as  the  muscle  is  seen  in  a  state  of  contraction  or  re- 
laxation. Still  further,  if  we  soak  a  fresh  muscle  for  twenty-four  hours 
in  a  half  per  cent,  solution  of  hydrochloric  acid,  and  then  tease  it,  we 
find  that  the  fibres,  instead  of  breaking  up  longitudinally  into  fibrillae, 
break  across  transversely  into  thin  disks.  We  thus  see  that,  by  break- 
ing up  in  these  two  directions,  the  whole  fibre  is  capable  of  being  re- 
solved into  a  multitude  of  tiny  prismatic  structures,  which  are  called 
sarcous  elements.  The  central  portion  of  each  prism  or  sarcous  ele- 
ment is  occupied  by  a  dark  portion,  while  at  each  end  is  a  lighter 
zone.  The  light  and  dark  zones  of  the  sarcous  elements,  when  the 
latter  are  grouped  together,  form  the  alternating  light  and  dark  bands 
of  the  fibres.  It  is  believed  by  many  observers  that  the  sarcous  ele- 
ments are  definite  and  independent  structures,  in  which  the  dark  por- 
tion is  the  contractile  element,  and  that  they  are  joined  together  side 
to  side  and  end  to  end  by  peculiar  cementing  substances. 

In  addition  to  these  markings  on  the  fibres,  if  high  magnifying 
powers  are  used  and  the  fibre  is  in  a  state  of  extension,  a  fine  line  is 
seen  crossing  the  fibre  through  the  centre  of  the  light  transverse  band  ; 
this  corresponds  with  the  dividing  line  between  the  ends  of  the  sar- 
cous elements,  and  is  called  Krause's  line.  Under  favorable  condi- 
tions the  dark  band  is  also  seen  to  be  crossed  by  a  line  called  Hen- 
serfs  line,  whose  nature  is  as  yet  but  imperfectly  explained. 

All  of  the  above  described  structural  features  of  the  muscular 
fibres  are  much  more  distinct  after  treatment  with  chemical  agents, 
and  after  death  of  the  tissue ;  the  longitudinal  striation  is  not  visible 
during  life,  and  the  distinct  separation  of  the  primitive  fibrillar  and 
disks  can  only  be  accomplished  by  chemical  means.  We  can  see  the 
various  markings  with  sufficient  clearness,  on  the  fresh  or  living  muscle, 
to  convince  ourselves  that  marked  optical  differences,  at  least,  exist 
in  different  parts  of  the  fibres  ;  but  that  the  living  fibre  is  made  up  of 
distinct  elements,  having  [the  structure  which  we  see  in  the  isolated, 
or  partially  isolated,  sarcous  elements,  although  probable,  is  by  no 
means  proven,  since  this  isolation  by  chemical  means  may  signify  only 
a  tendency  to  break  up  in  certain  directions,  and  not  a  definite,  pre- 
existing, independent  structure. 

2.  The  nuclei,  which  in  the  mammalia  lie  upon  the  surface  of  the 
fibres,  and  directly  beneath  the  sarcolemma,  in  the  amphibia,  fishes, 
and  certain  birds,  also  embedded  within  the  contractile  substance, 
are  usually  large,  flat,  and  ellipsoidal  in  shape,  contain  nucleoli,  and 
lie  with  their  long  axes  coincident  in  direction  with  the  axis  of  the 
fibres.  They  are  irregularly  scattered  along  the  fibre,  and  a  small 
amount  of  granular  matter  is  usually  seen  in  their  immediate  vicinity. 

3.  The  sarcolemma,  a  delicate,  structureless,  membranous  sheath, 
is  so  thin,  and  so  closely  encloses  the  contractile  substance  and  nu- 
clei, that  we  cannot  usually  see  it,  unless  we  separate  it  by  artificial 


$6  NORMAL   HISTOLOGY. 

means  from  the  underlying  structures.  Where  the  muscular  fibres 
join  tendons,  the  sarcolemma  ends  in  the  form  of  a  pointed  or  rounded 
blind  sac,  to  which  the  tendon-fibres  are  attached. 

The  muscular  fibres  lie  closely  packed  together,  their  ends  lapping 
over  onto  adjacent  fibres,  and  forming  bundles  which  are  enclosed  in 
sheaths  of  connective  tissue.  Such  bundles  are  again  grouped  to  form 
larger  bundles,  and  thus  the  larger  and  smaller  muscular  bellies  and 
bands  are  formed.  Arteries  enter  the  muscular  bundles  and  break 
up  into  capillaries,  which  run  along  the  fibres  forming  a  long  and  nar- 
row-meshed net.  Motor  nerves  also  pass  into  the  muscles,  divide  and 
subdivide,  and  terminate  at  the  surface  of  the  individual  fibres,  in 
structures  called  motor  end  plates. 


PRACTICAL    STUDY. 

Fresh  Muscle. — From  the  fresh  muscle  we  can  derive  a  tolerably 
clear  idea  of  most  of  the  above  described  structural  features. 

From  any  muscle  of  the  trunk  or  leg  of  a  rabbit — which  should 
have  been  dead  long  enough  to  allow  of  the  occurrence  of  the  rigor 
mortis,  since  the  contraction  of  the  fibres  which  occurs  in  perfectly 
fresh  muscle  obscures  the  pictures — a  small  bit  is  circumscribed  with 
a  sharp  scalpel,  and  a  fragment  dissected  off  with  as  little  stretching  as 
possible.  This  is  carried  at  once  to  a  drop  of  one-half  per  cent,  solu- 
tion of  salt,  on  a  slide,  and  here  carefully  teased,  the  fibres  being 
pulled  apart  from  the  ends  as  much  as  possible.  It  is  then  covered, 
a  bit  of  paper  or  hair  being  laid  beside  the  specimen  to  prevent  pres- 
sure from  the  cover-glass.  In  such  a  preparation  the  transverse  bands 
and  indistinct  longitudinal  striations  will  be  seen,  and  here  and  there, 
where  the  needles  have  pressed  on  the  fibres,  the  contractile  substance 
will  be  seen  to  have  been  broken  across  and  the  broken  ends  to  have 
retracted  within  the  sarcolemma,  leaving  the  latter  as  a  clear  and 
sometimes  folded  membrane  stretching  across  the  interval.  At  the 
cut  ends  of  the  fibres,  the  contractile  substance  will  often  be  seen 
swelling  and  extruding  from  the  sarcolemma,  in  the  form  of  an  ob- 
scurely striated  fungiform  mass.  Here  and  there  nuclei  are  seen  ;  but 
they  may  be  brought  much  more  clearly  into  view,  by  allowing  a  drop 
of  two  per  cent,  acetic  acid  to  flow  under  the  cover-glass ;  then  the 
contractile  substance  swells  and  becomes  transparent,  the  striations 
becoming  indistinct  as  the  somewhat  shrunken  nuclei  become  more 
clearly  defined. 

Sections  of  Hardened  Muscle. — The  details  of  the  structure  of 
muscular  fibres,  as  well  as  their  grouping  and  relation  to  the  connec- 
tive tissue,  may  be  well  studied  in  sections  from  hardened  muscle. 
For  this  purpose  the  tongue  of  some  animal — such  as  the  dog— is  well 
suited,  since  here  we  have  short  muscular  fibres  running  in  various 
directions  and  attached  to  tendons,  and  we  see  in  a  single  transverse 
section  of  the  organ,  at  once,  longitudinal  and  transverse  sections  of 
the  fibres.  The  tongue  of  a  recently  killed  animal  is  cut  transversely 
across  the  middle,  and  put,  for  ten  days,  in  a  two  per  cent,  solution  of 
bichromate  of  potassium  ;  then  soaked  in  water  for  an  hour,  and  trans- 


MUSCULAR  TISSUE.  57 

ferred  to  alcohol,  in  which  it  lies  till  sufficiently  hard.  Thin  transverse 
sections  are  now  made,  including  a  whole  or  a  portion  of  the  organ, 
and  stained  with  hematoxylin  and  mounted  in  glycerine.  The  trans- 
verse bands,  with  their  accompanying  lines,  are  best  seen  in  unstained 
sections,  mounted  in  water,  since  the  coloring  somewhat  obscures  the 
finer  structure,  and  glycerine,  owing  to  its  high  refractive  power, 
renders  them  less  distinct.  To  preserve  the  unstained  specimens 
permanently,  however,  glycerine  must  be  used. 

In  muscular  tissue,  hardened  as  above  in  bichromate  of  potassium, 
the  primitive  fibrillar  are  loosened  from  each  other,  and  in  some  parts  of 
the  specimen  are  usually  separated  by  the  manipulation  of  the  section. 

Blood-vessels  are  seen  in  the  above  preparations,  but  they  may  be 
much  better  demonstrated  in  sections  of  muscular  tissue  whose  vessels 
have  been  injected  with  some  colored  material,  such  as  the  blue  gela- 
tine. 

i.  b. — Involuntary  Striated  or  Heart  Muscle. 

In  mammalia,  the  heart-muscle  differs  in  several  important  struc- 
tural features  from  the  voluntary  muscle. 

The  contractile  substance  has  essentially  the  same  structure  as  the 
latter,  but,  instead  of  being  arranged  in  the  form  of  elongated,  un- 
branched  cylinders  or  fibres,  without  distinct  cell-structure,  in  the 
heart-muscle  the  fibres  send  off  at  frequent  intervals  short,  narrow 
processes,  which  join  neighboring  fibres,  forming  a  narrow  and  long 
meshed  net.  Further,  the  fibres  which,  owing  to  the  numerous  anas- 
tomoses, are  very  irregular  in  form,  are  made  up  of  distinct  segments 
or  cells,  each  segment  being  cemented  at  the  ends  to  its  neighbors, 
and  furnished  with  a  flat,  elongated,  ovoidal,  or  often  rectangular  nu- 
cleus. In  the  vicinity  of  the  nuclei  we  usually  see  a  certain  amount 
of  granular  material  or  pigment  particles.  The  nuclei,  instead  of 
lying,  as  in  the  voluntary  muscles,  at  the  surface  of  the  contractile  sub- 
stance, are  embedded  within  it.  We  are  unable,  in  heart-muscle,  to 
demonstrate  a  sarcolemma.  The  fibres  are  grouped  in  bundles,  which 
are  enclosed  in  connective  tissue,  and  supplied  with  blood-vessels  and 
nerves. 

PRACTICAL    STUDY. 

Sections  of  Heart-Muscle. — A  bit  of  the  heart  of  man,  or  any 
mammal,  should  be  hardened  first  in  Miiller's  fluid,  and  then  in  alcohol. 
Longitudinal  and  transverse  sections  are  made,  stained  with  haema- 
toxylin, and  mounted  in  glycerine. 

Teased  Heart-Muscle. — To  demonstrate  the  segments  or  cells  of 
which  the  heart-fibres  are  composed,  a  moderately  thin  bit  of  fresh 
heart-muscle  is  put  for  twenty-four  hours  into  a  dilute  solution  of 
chromic  acid  (i  to  5,000),  then  carefully  washed  with  pure  water  and 
stained  with  picro-carmine.  It  is  then  teased  on  a  slide  and  mounted 
in  glycerine,  to  which  a  little  formic  acid  has  been  added  (1  to  100). 
The  formic  acid  renders  the  tissue  more  transparent,  and  brings  more 
clearly  into  view  the  narrow,  jagged  lines  of  strongly  refractive  sub- 
stance which  indicate  the  boundaries  of  the  muscle-cells. 


CHAPTER    VII. 

NERVE-TISSUE. 

The  primary  structural  element  in  nerve-tissue  is  the  nerve-cell. 
Nerve-cells  have  the  most  diverse  forms,  and  always  possess  one  or 
more  branching  or  unbranched  processes.  In  certain  cells  the  un- 
branched  processes  are  extremely  long,  become  associated  with  other 
tissue  elements,  and  constitute  the  nerve-fibres.  Both  nerve-cells 
and  their  processes,  nerve-fibres,  are  enclosed  and  supported  by 
peculiarly  arranged  connective  tissue,  and  supplied  with  blood  and 
lymphatic  vessels.  The  nerve-fibres  form,  for  the  most  part,  the 
white  matter  of  the  nervous  centres  and  the  peripheral  nerves,  while 
the  cells  enter  largely  into  the  composition  of  the  gray  matter.  In 
studying  nerve-tissue,  we  have  then  to  consider  : 

I.  Nerve-fibres^  and  the  supporting  connective-tissue  structures, 
with  their  accessories. 

II.  Nerve-cells. 

I.  Nerve-fibres,  etc. 

These  are  of  two  kinds  :  a,  medullated,  and  b.  non-medullated. 
This  distinction  corresponds  with  the  physiological  and  anatomical 
classification  of  nerve-tissues  into  those  of  the  cerebro-spinal  and  the 

sympathetic  systems  :  the  medullated  belonging  to  the  former,  the 
non-medullated  to  the  latter. 

a.  Medullated  Nerve-fibres. — If  we  disregard,  for  the  moment,  the 
structure  of  these  nerves  at  their  point  of  origin  in  the  nerve-centres, 
and  at  their  termination  in  the  periphery,  and  study  their  structure  as 
it  is  seen  in  the  continuity  of  any  of  the  larger  or  smaller  nerves,  we 
hnd  that  the  individual  fibres  present  three  distinct  structural  elements  : 
i,  the  axis  cyli?ider  ;  2,  the  medullary  sheath  ;  3,  the  ?ieurilemma. 

1.  Running  through  the  axis  of  the  fibre  is  a  cylindrical,  with  high 
powers,  delicately  longitudinally  striated  structure — the  axis  cylinder. 
This  is  believed  to  be  the  essential  nerve-element  of  the  fibre — the 
process  of  the  nerve-cell,  from  which  it  passes  without  break  of 
continuity  to  the  periphery ;  and  it  is  probable  that  the  longitudinal 
striations  are  the  expression  of  its  composition  from  still  finer  primi- 
tive fibrils,  which,  as  we  shall  see  when  we  study  the  nerve-cells,  seem 
to  be  continued  on  into  the  cell-body  itself  within  the  nerve-centres. 

2.  Closely  surrounding  the  axis  cylinder  is  a  tube  or  sheath  of 
varying  thickness — the  medullary  sheath — composed  of  a  white,  semi- 
fluid, translucent,  strongly  refractive  substance,  called  myeline,  which 


NERVE-TISSUE.  59 

undergoes  rapid  changes  after  death,  or  on  removal  from  the  animal, 
and  swells  and  assumes  a  multitude  of  bizarre  forms  on  addition  of 
water ;  it  is  soluble  in  alcohol,  chloroform  and  ether ;  and,  like  fat,  is 
hardened  and  turned  black  under  the  action  of  osmic  acid.  The 
medullary  sheath  does  not  form  a  continuous  tube,  but  at  tolerably 
regular  intervals  is  separated  into  segments. 

3.  The  neurilemma  or  sheath  of  Schwann  is  an  extremely  thin, 
structureless,  membranous  tube,  which  tightly  encloses  the  medullary 
sheath,  and,  like  the  latter,  is  broken  up  into  segments.  At  the  ends 
of  the  segments  is  a  constriction  around  the  fibre,  at  the  expense  of 
the  medullary  sheath,  and  the  ends  of  the  neurilemma  segments  are 
joined  together  by  a  thin  layer  of  cement  substance,  which  extends 
inward  to  the  axis  cylinder.  There  is  reason  to  believe  that  the  neu- 
rilemma extends  inward  and  between  the  medullary  sheath  and  the 
axis  cylinder,  entirely  enclosing  the  segments  of  the  medullary  sheath. 

Within  each  neurilemma  segment,  called  interannular  segment, 
and  about  midway  between  the  constrictions,  lies  a  flattened,  elon- 
gated, elliptical  nucleus.  We  may  regard  the  neurilemma  segments, 
with  their  nuclei,  as  cylindrical  cells,  cemented  together,  end  to  end, 
enclosing  the  segments  of  the  medullary  sheath,  and  surrounding  the 
axis  cylinder,  the  latter  passing  uninterruptedly  through  the  axis  of 
the  segments.  In  addition  to  these  structural  features,  we  find,  on 
examining  with  high  powers,  irregularly  scattered  along  each  inter- 
annular segment,  delicate  oblique  lines  or  fissures,  called  the  incisures 
of  Schmidt,  which  seem  to  pass  from  the  neurilemma  at  the  surface 
inward  to  the  axis  cylinder,  obliquely  through  the  medullary  sheath. 
Their  significance  is  not,  as  yet,  definitely  determined.  Medullated 
nerve-fibres  vary  greatly  in  diameter. 

Connective  Tissue  of  the  Nerves. — The  nerve-fibres  are  bound  to- 
gether by  connective  tissue,  to  form  larger  and  smaller  nerve-fascicles, 
which,  singly  or  in  bundles,  we  usually  call  simply  nerves.  If  we  fol- 
low the  nerves  outward  toward  their  peripheral  terminations,  we  find 
that  they  divide  and  subdivide,  becoming,  as  they  do  so,  smaller  and 
smaller,  until  we  finally  come  to  nerves  which  consist  of  a  single 
fibre.  These  single  nerve-fibres  do  not  lie  free  in  the  tissues,  but  are 
enclosed  in  a  distinct  sheath,  called  Henle's  sheath,  which  is  a  tube 
formed  of  a  single  layer  of  endothelial  cells,  placed  edge  to  edge,  and 
cemented  together.  Between  the  sheath  and  the  nerve-fibre  is  a  nar- 
row space,  which,  under  normal  conditions,  is  filled  with  lymph. 

Having  become  acquainted  with  this  simple  structure  of  the  single 
terminal  nerves,  let  us  follow  them  backward.  We  find,  as  we  do  so, 
that,  as  they  become  larger  by  the  junction  of  several  fibres,  a  small 
amount  of  fibrillar  connective  tissue  appears  between  the  fibres  within 
Henle's  sheath,  and  that  the  latter  becomes  attached  to  the  surround- 
ing structures  by  a  layer  of  connective  tissue.  Finally,  when  we  ar- 
rive at  the  larger  nerve-trunks,  we  find  that  in  each  the  connective 
tissue  presents  itself  in  three  ways  : 

1.  It  forms  a  distinct  sheath,  which,  although  the  analogue  of 
Henle's  sheath,  has  a  much  more  complicated  structure,  and  is  called 
the  lamellar  sheath.     This  is  composed  of  several  concentric  lamellae, 


CO  NORMAL   HISTOLOGY. 

each  of  which  is  formed  of  a  fenestrated  membrane  of  fibrillar  connec- 
tive tissue  containing  granules  of  elastic  tissue  substance,  and  covered 
with  endothelial  cells.  The  lamellae  are  connected  by  oblique  fibres, 
which  pass  from  one  to  the  other,  binding  them  more  or  less  firmly 
together.  The  whole  forms  a  compact  sheath,  closely  investing  the 
fascicle  of  nerve-fibres. 

2.  Outside  the  lamellar  sheath,  and  joining  it  to  adjacent  struc- 
tures— with  neighboring  fascicles,  if  the  nerve-trunk  is  composed  of 
several  of  these,  as  is  the  case  in  many  large  nerves — we  find  loose 
fibrillar  connective  tissue  with  flattened,  irregular-shaped  cells — like 
those  found  in  the  loose  subcutaneous  connective  tissue — reinforced 
by  elastic  fibres,  and  often  containing  fat-cells.  The  fibrillated  and 
elastic  fibres,  especially  in  the  immediate  vicinity  of  the  lamellar 
sheath,  usually  run  in  a  direction  approximately  parallel  with  the  axis 
of  the  nerve.  This  tissue  is  called  the  perifascicular  connective 
tissue. 

3.  We  find  within  the  lamellar  sheath  and  between  the  nerve-fibres 
composing  the  fascicle,  in  the  first  place,  prolongations  inward  of  the 
membranous  tissue  composing  the  lamellar  sheath  ;  and  second,  fine 
fibrillated  fibres  and  flattened  cells  which  lie  in  the  interstices  between 
the  nerves  and  fibres.  This  tissue  is  called  the  inlrafascictUar  con- 
nective tissue.  Blood-vessels  penetrate  the  lamellar  sheath  of  the 
medium-sized  and  larger  nerves,  and  a  very  long  meshed  and  abundant 
capillary  net-work  is  formed  in  the  intrafascicular  connective  tissue. 
Lymphatic  channels  and  spaces  are  also  abundant  within  the  nerves, 
so  that  the  fibres  are  bathed  in  nutritive  fluids. 

Termination  of  Medullated  Nerve -fibres. 

1.  In  the  .Nerve-centres. — We  find  in  the  nerve-centres  nerves  which 
have  no  neurilemma,  and  others  in  which  both  neurilemma  and  med- 
ullary sheath  fail — the  so-called  naked  axis  cylinders  ;  we  find,  further, 
extremely  delicate  filiform  structures  which  are  believed  to  be  the 
primitive  nerve-fibrils. 

The  axis  cylinders  of  the  nerves  being,  as  above  stated,  processes 
of  nerve-cells,  the  nerve-fibres,  as  we  trace  them  back  into  the  centres, 
must  sooner  or  later  join  cells.  Their  exact  mode  of  connection  with 
the  cells  is  not  in  all  cases  sufficiently  well  understood  ;  but  it  is  be- 
lieved that  they  either  join  the  cells  in  the  form  of  naked  axis  cylin- 
ders— as  we  shall  see  when  we  study  the  nerve-cells — or,  in  other  cases, 
that  the  axis  cylinders  break  up  into  their  constituent  primitive  fibrils 
before  entering  the  cells. 

2.  In  the  Periphery. — The  peripheral  termination  of  nerves  is  a 
subject  which  presents  extreme  difficulties  to  the  histologist,  and  with 
few  exceptions  the  exact  way  in  which  this  occurs  is  unknown  ;  and, 
since  a  consideration  of  the  subject  in  detail  does  not  lie  within  the 
scope  of  these  lessons,  we  shall  here  simply  describe  one  of  the  best 
understood  modes  of  termination — that  in  voluntary  muscular  fibre. 
Some  of  the  other  modes  will  be  briefly  described  as  we  study,  in  sub- 
sequent lessons,  the  parts  and  organs  in  which  they  occur. 


NERVE-TISSUE.  6 1 

Termination  of  Nerves  in  Voluntary  Muscle. — If  we  trace  a  nerve  as 
it  passes  into  a  muscular  bundle,  we  find  that  it  divides  and  subdivides, 
and  finally  we  find  a  single  fibre  within  its  Henle's  sheath  suddenly 
piercing  the  sarcolemma  at  the  surface  of  a  muscular  fibre  and  going 
out  of  sight. 

By  various  modes  of  preparation  we  learn  that  Henle's  sheath  be- 
comes continuous  with  the  sarcolemma  ;  the  medullary  sheath  disap- 
pears, and  the  axis  cylinder,  and  possibly  the  neurilemma,  pass  beneath 
the  sarcolemma.  Here  the  axis  cylinder  usually  divides  into  several 
irregular,  often  knobbed  branches,  which  lie  and  apparently  terminate  in 
a  flattened,  irregular-shaped  mass  of  granular  substance,  which  is  fur- 
nished with  variously  shaped  nuclei.  This  structure  is  called  the 
motor  end  plate.  It  is  probable  that  each  muscular  fibre  is  furnished 
with  at  least  one  of  these  terminal  apparatuses.  The  above  descrip- 
tion applies  to  the  mammalia.  In  certain  of  the  lower  animals,  as  the 
frog,  the  termination  is  somewhat  different. 

b.  Non-medullated  Nerve-fibres. — These  are  also  called  fibres  of 
Remak.  Unlike  the  nerve-fibres  which  we  have  just  been  studying, 
they  possess  no  medullary  sheath  and  no  neurilemma.  They  are 
simply  grayish  translucent  cords  of  varying  diameter,  are  indistinctly 
longitudinally  striated,  and  are  intimately  connected  with  one  another 
by  frequent  inosculations.  The  fibres  seem  to  divide  and  send  off 
oblique  branches  to  join  neighboring  fibres.  Flattened,  elongated 
nuclei  lie  at  frequent  intervals  upon  the  surface  of  the  fibres.  These 
fibres  considerably  resemble,  in  their  general  appearance,  the  fibril- 
lated  fibres  of  ordinary  connective  tissue,  but  careful  examination 
shows  them  to  be  entirely  distinct  structures. 

They  are  grouped  in  bundles  to  form  nerves,  sometimes  alone,  but 
very  frequently  in  connection  with  medullated  nerve-fibres.  Thus,  in 
the  pneumogastric,  we  find  a  considerable  part  of  the  fibres  to  be  non- 
medullated  and  intimately  bound  in  by  the  intrafascicular  connective 
tissue,  with  medullated  fibres.  The  non-medullated  fibres  originate  in 
nerve-cells  of  a  peculiar  structure,  to  be  presently  described  :  but  of 
their  peripheral  terminations  we  know  almost  nothing. 

II.  Nerve-cells. 

Nerve-cells,  or  ganglion-cells,  as  they  are  frequently  called,  al- 
though presenting  the  greatest  diversity  in  form,  have  yet  some  quite 
distinctive  characters  in  common.  The  cell-body  is  finely  granular 
and  delicately  striated,  often  containing  pigment-granules.  The 
nucleus  is  large,  well-defined,  vesicular  in  appearance,  and  usually 
contains  a  large  shining  nucleolus.  They  all  have  at  least  one  pro- 
cess, most  of  them  have  more  ;  and  they  are  hence  often  classified  as 
unipolar,  bipolar,  or  multipolar  ganglion-cells.  The  above-mentioned 
striations  in  the  cell-body  are  often  seen  to  continue  out  into  the  pro- 
cesses. In  many  nerve-cells,  especially  in  the  spinal  cord,  we  recog- 
nize two  distinct  kinds  of  processes  :  first,  those  which,  soon  after 
leaving  the  cell,  divide  and  subdivide  until  they  become  extremely 
fine  and  delicate,  and,  in  some  cases,  seem  to  join  equally  fine  pro- 


62  NORMAL   HISTOLOGY. 

cesses  of  other  cells — such  delicate  cell-processes  make  up  a  consider- 
able portion  of  the  gray  matter  of  the  cord,  and  are  called  branch- 
ing processes  ;  second,  such  as  pass  off  from  the  cell,  and,  without 
dividing,  presently  are  surrounded  by  a  sheath  of  myeline,  and  become 
medullated  nerve-fibres  ;  the  latter  are  called  axis-cylinder  processes. 
Nerve-cells  vary  greatly  in  size,  and,  although  the  forms  which  they 
present  are  most  diverse,  we  yet  find  that  a  considerable  proportion 
of  those  found  in  different  parts  of  the  nerve-centres  have  certain 
broadly  typical  forms.  Thus,  among  the  cells  in  the  gray  matter  of 
the  spinal  cord,  we  find  larger  and  smaller  fusiform  or  spheroidal 
branching  cells,  or.  which  are  more  characteristic,  large,  irregular- 
shaped  cells,  with  several  branching  processes  and  a  well-defined  axis- 
cylinder  process.  In  the  cortex  of  the  cerebrum,  while  we  find  vari- 
ously shaped  larger  and  smaller  cells,  we  find  also  characteristic  pyra- 
midal cells  of  varying  size,  which  give  off  processes  from  both  the 
base  and  apex. 

In  the  cerebellum,  we  find  just  at  the  inner  edge  of  the  gray  corti- 
cal matter,  irregular  globular  or  ovoidal  cells,  which  from  the  side  to- 
ward the  surface  of  the  brain,  send  off  one  or  two  branching  processes  ; 
on  the  opposite  side  we  can  usually  demonstrate  the  commencement 
of  a  single  delicate  process,  which  is  supposed  to  correspond  to  the 
axis-cylinder  process,  though  since  it  almost  invariably  breaks  off  near 
the  cell  in  the  attempt  to  isolate  the  latter,  its  nature  is  not  yet  defi- 
nitely determined.     These  cells  are  called  Purkinje's  cells. 

The  ganglion-cells  of  the  sympathetic  are  usually  globular  or  ovoidal, 
and  are  peculiar  in  that  each  cell  is  surrounded  by  a  distinct  capsule 
of  connective  tissue,  lined  with  flattened  cells,  resembling  endothe- 
lium. 

They  have  usually  but  one  process — sometimes  two — which  pierces 
the  capsule  and  becomes  a  non-medullated  nerve-fibre. 

PRACTICAL    STUDY. 

Fresh  Nerve. — A  bit  of  fresh  nerve — the  sciatic  of  the  frog  answers 
very  well— should  be  carefully  and  rapidly  teased  apart  longitudinally,  in 
one-half  per  cent,  salt  solution — care  being  taken  to  pull  apart  the  fibres 
from  the  ends,  so  as  to  break  them  as  little  as  possible — and  covered  ; 
pressure  from  the  cover-glass  being  avoided  by  placing  a  bit  of  paper  or 
hair  beside  the  specimen.  The  nerve-fibres  present  if  examined  at  once, 
in  many  parts,  a  sharp  and  regular  double  contour,  which  is  their  nor- 
mal appearance,  and  along  their  course  the  constrictions  and  nuclei 
may  here  and  there  be  seen.  The  axis  cylinder  and  neurilemma  are 
for 'the  most  part  invisible,  the  former  owing  to  the  lack  of  transpar- 
ency in  the  medullary  sheath,  the  latter  because  of  its  extreme  thin- 
ness and  close  contact  with  the  medullary  sheath.  Very  soon,  at  once 
in  some  parts  of  the  specimen,  the  contours  of  the  fibres  will  be  seen 
to  become  irregular,  the  myeline  shrinking  away  at  some  parts  from 
the  neurilemma  and  swelling  out  at  others.  At  the  severed  ends  of 
the  fibres  the  myeline  will  be  seen  welling  out  from  the  neurilemma, 
and  breaking  off  into  the  fluid  in  irregular  globular  or  contorted  masses. 


NERVE-TISSUE.  6$ 

After  the  swelling  and  irregular  breaking  up  of  the  myeline  has  oc- 
curred— this  may  be  hastened  by  allowing  water  to  run  under  the  cover- 
glass — the  neurilemma  may  be  seen  here  and  there  stretching  across 
between  the  varicosities  formed  by  the  swollen  myeline,  and  either  at 
the  broken  ends  of  the  fibres  or  along  their  course  the  axis  cylinder 
may  occasionally  be  seen. 

Demonstration  of  the  Axis  Cylinder  by  Collodion. — In  order  to  ob- 
tain a  clear  view  of  the  axis  cylinder,  we  should  tease  a  fresh  nerve, 
without  the  addition  of  any  fluid,  and  when  the  natural  fluid  of  the  tis- 
sue has  partially  evaporated — care  being  taken  not  to  allow  it  to  be- 
come quite  dry — a  drop  of  collodion  should  be  added  and  the  speci- 
men quickly  covered  and  examined  at  once. 

The  collodion  dissolves,  or  renders  the  myeline  transparent,  and 
the  axis  cylinder  is  distinctly  seen  within  the  neurilemma,  either  in  the 
axis  of  the  fibre  or  along  one  side. 

Nerve-fibres  Treated  with  Osmic  Acid. — The  most  complete  dem- 
onstration of  the  nerve-fibre  in  its  entirety  may  be  obtained  by  treat- 
ment with  osmic  acid.  This  agent  fixes  the  myeline  and  other  con- 
stituents of  the  fibre  nearly  in  their  normal  form,  staining  the  myeline 
black.  In  applying  this  agent  it  is  necessary  to  maintain  the  nerve  in 
a  state  of  gentle  tension,  because  it  is  otherwise  somewhat  contracted 
and  distorted.  This  is  done  by  gently  stretching  the  nerve — the  sci- 
atic of  the  rabbit  will  answer — along  a  bit  of  wood  which  has  been 
whittled  away  at  one  side,  so  that  the  nerve  may  lie  free.  It  is  fas- 
tened by  the  ends  to  the  wood  by  threads.  The  nerve  thus  prepared 
is  immersed  for  twenty-four  hours  in  an  aqueous  solution  of  osmic  acid 
(i  to  ioo),  then  washed,  and  a  small  bit  carefully  teased  apart  longi- 
tudinally in  glycerine.  In  such  a  preparation  nearly  all  the  struc- 
tures in  the  fibre  can  be  readily  seen  :  the  constrictions  and  nuclei,  the 
medullary  sheath  stained  black,  the  incisures  of  Schmidt,  and  where, 
as  will  almost  always  occur  in  some  parts  of  the  specimen,  the  medul- 
lary sheath  has  been  broken  across  or  the  segments  pulled  asunder,  or 
the  myeline  has  contracted  at  the  constrictions,  the  neurilemma,  and 
axis-cylinder.  Not  infrequently,  if  the  teasing  has  not  been  very  care- 
fully done,  the  segments  of  the  medullary  sheath  are  broken  across  in 
many  places  and  separated,  giving  the  fibre  a  beaded  appearance. 

Transverse  sections  of  nerves  stained  with  osmic  acid  give  instruc- 
tive pictures.  A  nerve  treated  as  above  with  osmic  acid  is  not  firm 
enough  to  permit  the  making  of  thin  sections,  and  should  be  hard- 
ened by  what  is  known  as  the  gum  embedding  method,  as  follows  :  a 
bit  of  the  nerve,  after  the  osmic  acid  treatment,  is  immersed  for  twen- 
ty-four hours  in  alcohol,  washed,  and  immersed  for  twenty-four  hours 
in  a  syrupy  solution  of  gum-arabic,  then  put  again  for  twenty-four 
hours  in  alcohol.  By  this  process  the  gum  is  deposited  in  solid  form 
in  the  interstices  of  the  fibres,  giving  it  sufficient  consistence  for  cutting. 
The  sections,  either  stained  with  picro-carmine  or  unstained,  are 
mounted  in  glycerine.  In  such  a  preparation  the  uncolored  axis 
cylinder  is  seen  surrounded  by  a  black  ring,  the  medullary  sheath,  and 
here  and  there  the  nuclei  of  the  neurilemma  are  seen  at  the  edge  of  the 
fibres.     The   connective  tissue  surrounding  the   fibres  has   a  grayish 


64  NORMAL   HISTOLOGY. 

color.  The  fibres  will  be  seen  to  have  varying  diameters  and  to  pre- 
sent marked  differences  in  form,  some  of  these  depending  upon  artifi- 
cial changes,  others  upon  the  difference  in  level  at  which  the  fibres  have 
been  cut  across. 

Transverse  Sectio?is  of  Nerves  Preserved  in  Chromic  Acid. — Bits 
of  the  sciatic  nerve  from  the  rabbit  or  any  other  mammal  should  be 
lightly  stretched  along  a  bit  of  wood  and  placed  in  a  solution  of  chro- 
mic acid  (i  to  500);  in  two  weeks  it  is  washed  and  transferred  to  alco- 
hol. It  is  now  embedded  in  gum  (see  above),  and  thin  transverse  sec- 
tions made  and  stained  with  hematoxylin  and  eosin  and  mounted  in 
balsam.  In  such  preparations  the  general  relations  of  connective 
tissue  to  the  nerve-fibres  is  well  seen. 

Nerve-fibres  Treated  with  Nitrate  of  Silver. — By  this  method  we 
obtain  hints  concerning  the  structure  of  nerves  which  are  of  no  little 
significance  from  a  physiological  point  of  view.  A  fresh  nerve  is 
slightly  teased  apart  on  a  slide,  a  large  drop  of  a  one-half  per  cent, 
solution  of  nitrate  of  silver  added  and  allowed  to  remain  for  four  min- 
utes ;  this  is  washed  off  with  one-half  per  cent,  salt  solution,  the  speci- 
men transferred  to  a  drop  of  glycerine  on  another  slide,  the  fibres 
carefully  teased  apart,  and  covered.  The  preparation  is  now  exposed 
to  sunlight  or  diffuse  daylight  until  it  becomes  brown.  If  now  exam- 
ined, at  tolerably  regular  intervals  along  the  fibre  tiny  brown  or  black 
crosses,  called  Ranviefs  crosses ,  will  be  seen,  the  transverse  arm  of  the 
cross  being  the  stained  cement  substance  between  the  neurilemma  seg- 
ments at  the  constrictions  ;  the  longitudinal  arm,  which  coincides  with 
the  axis  of  the  fibre  and  which  is  longer  or  shorter,  depending  upon 
the  length  of  time  to  which  the  fibre  was  exposed  to  the  action  of  the 
silver,  is  the  axis  cylinder.  If  a  specimen  be  allowed  to  remain  longer 
than  the  above  time  in  contact  with  the  silver,  the  longitudinal  arm  of 
the  cross  will  be  longer  ;  and  if  the  contact  be  continued  for  a  suffi- 
cient time,  the  axis  cylinder  will  be  stained  black  along  the  whole 
length  of  the  fibre.  It  is  to  be  observed  that  the  axis  cylinder  is  first 
stained  at  that  part  which  passes  through  the  constrictions,  and  not 
along  the  segments.  Certain  other  soluble  substances  which  stain  the 
axis  cylinder  comport  themselves  in  the  same  way.  We  infer  from 
this  that  at  the  constrictions  certain  substances  in  solution  can  pass 
into  the  fibre  and  come  in  contact  with  the  axis  cylinder  or  the  nerve- 
element  proper  of  the  fibre.  This  inference  is  significant  in  connec- 
tion with  the  nutrition  of  the  nerves,  since  we  are  justified  in  assuming 
that  nutritive  substances  in  solution  may  pass  also  to  the  axis  cylinder 
in  the  same  way. 

It  is  not  improbable  that  the  constrictions  serve  yet  another  im- 
portant purpose.  The  myeline  of  the  medullary  sheath  being  a  semi- 
fluid substance — perhaps  serving  either  to  isolate  the  axis  cylinder  or 
protect  it  from  external  violence — it  would  inevitably  tend  to  gravitate 
to  the  lower  parts  of  the  nerves,  were  it  not  that  it  is  held  in  position 
by  being  enclosed,  so  to  say,  in  cylindrical  cases,  i.  e.,  the  neurilemma- 
cells,  between  the  constrictions. 

Naked  Axis  Cylinders  in  the  Spinal  Cord. — To  demonstrate  these, 
a  perfectly  fresh  bit  of  the  white  substance  of  the  spinal  cord  is  put  for 


NERVE-TISSUE.  65 

twenty-four  hours  in  a  one  per  cent,  solution  of  osmic  acid,  and  teased 
in  glycerine.  In  addition  to  the  naked  axis  cylinders  and  medullated 
fibres,  free  globules  of  myeline  and  fragments  of  connective  tissue  and 
blood-vessels  will  be  seen. 

The  non-medullated  nerve-fibres  may  be  demonstrated  in  connection 
with  the  sympathetic  ganglion- cells. — See  below. 

1.  Nerve-cells :  a.  Spinal  Cord. — Small  bits  of  the  gray  matter  from 
the  spinal  cord  of  man,  or  from  the  ox  or  sheep,  should  be  put  for  ten 
days  in  a  dilute  solution  of  chromic  acid  (1  to  500),  and  then  carefully 
shaken  in  a  test-tube  with  water  colored  with  carmine  ;  when  the  bits 
have  become  thoroughly  broken  up  into  small  particles,  the  tube  is 
allowed  to  stand  for  a  day  or  two,  until  the  particles  which  have  set- 
tled to  the  bottom  are  sufficiently  stained.  The  supernatant  fluid  is 
then  decanted,  and  with  a  glass  tube  a  small  drop  of  the  disintegrated 
tissue  is  conveyed  to  a  drop  of  glycerine  on  a  slide  and  covered,  pres- 
sure on  the  cells  being  avoided  in  the  usual  way.  If  the  first  prepara- 
tion does  not  contain  the  required  cells,  others  should  be  made.  In 
this  way,  if  the  shaking  be  carefully  done,  the  ganglion-cells  are  freed 
to  a  considerable  degree  from  the  surrounding  parts,  and  such  may  be 
found  as  present  numerous  long,  branching  processes  as  well  as  the 
axis-cylinder  process.  In  addition  to  the  cells,  such  specimens  pre- 
sent fragments  of  connective  tissue,  bits  of  naked  axis  cylinders,  and 
medullated  nerve-fibres,  myeline-droplets,  etc. 

b.  Brain. — In  the  way  above  described,  cells  should  be  prepared 
from  the  gray  cortical  portion  of  the  cerebrum  and  cerebellum. 

c.  Sympathetic. — For  the  demonstration  of  these  cells  and  the 
fibres  connected  with  them,  the  frog  answers  very  well.  The  animal 
having  been  killed  by  breaking  up  the  medulla,  the  abdominal  cavity 
is  opened  and  the  intestines  and  liver  carefully  removed  ;  the  aorta 
will  then  be  seen  lying  along  the  vertebral  column.  The  sympathetic 
ganglia  and  nerves  lie  along  the  walls  of  the  aorta  and  in  the  tissue 
surrounding  the  origin  of  the  spinal  nerves.  The  head  and  fore-legs 
should  now  be  cut  off  close  behind  the  latter,  and  the  hind-legs  sev- 
ered close  to  the  body  ;  the  trunk  is  then  laid  in  a  small  dish  and  cov- 
ered with  equal  parts  of  one  per  cent,  solution  of  osmic  acid,  alcohol, 
and  water.  The  dish  is  covered  and  set  aside  for  twenty-four  hours, 
when  the  aorta,  together  with  the  tissue  surrounding  the  commence- 
ment of  the  spinal  nerves,  is  dissected  off  in  a  single  piece,  spread  on 
a  slide,  and  examined  with  a  low  power.  Groups  of  sympathetic 
nerve-ceils  are  seen  here  and  there  in  the  specimen,  and  are  readily 
distinguished  by  the  orange  color  of  the  cells  ;  one  or  two  of  them  are 
to  be  isolated  and  freed  as  much  as  possible  from  the  enclosing  tissue, 
carefully  teased  apart  on  a  slide,  stained  lightly  with  hematoxylin  and 
then  with  eosin,  and  mounted  in  glycerine.  Successful  preparations 
will  show  not  only  the  nerve-cells  and  fibres,  but  also  the  connection 
of  the  two  within  the  capsule. 

5 


CHAPTER   VIII. 

BLOOD-VESSELS— LYMPHATIC    VESSELS. 

Blood-vessels. 

Blood-vessels  are  of  three  kinds  :  arteries,  veins,  and  capillaries. 
Although  merging  without  sharp  demarcation  into  one  another,  these 
vessels,  in  their  typical  forms,  present  sufficient  differences  in  struc- 
ture to  justify  their  separate  consideration.  The  capillaries  being  the 
simplest,  it  will  be  convenient  to  commence  with  them.  If  we  exam- 
ine a  capillary  vessel,  either  fresh  or  after  it  has  been  in  preservative 
fluids,  it  presents  the  appearance  of  a  narrow  tube,  with  very  thin, 
homogeneous  walls,  in  which,  at  frequent  intervals,  elongated  nuclei 
are  embedded,  their  long  axis  being  parallel  with  the  axis  of  the  tube. 
W,  however,  we  inject  the  vessels  with  a  dilute  solution  of  nitrate  of 
silver  and  expose  them  to  the  light,  we  find  that  the  inside  of  the  tube 
is  divided  by  narrow  black  lines  into  elongated,  irregular-shaped 
spaces  ;  and  if  we  then  stain  the  specimen  with  hematoxylin,  we  find 
that  a  nucleus  lies  in  each  space.  The  walls  of  the  capillaries  are, 
then,  not  formed  by  a  homogeneous  membrane,  but  made  up  of  cells 
having  the  character  of  endothelium.  The  capillaries  are  endothelial 
tubes.  This  layer  of  endothelial  cells,  which  alone  forms  the  walls  of 
the  capillaries,  is  found  lining  all  the  other  blood  channels,  arteries, 
and  veins,  as  well  as  the  heart.  In  the  blood-vessels  it  is  called  the 
intima.  If,  now,  we  follow  the  capillaries  in  a  direction  toward  the 
arteries,  we  find  that  the  connective  tissue  in  which  they  lie  is  arranged 
in  the  form  of  a  thin  layer  along  their  walls.  This  layer,  which  is 
also  present  in  all  arteries  and  veins,  is  called  the  adventitia. 

Almost  as  soon  as  we  find  the  adventitia,  we  notice  another  layer 
between  it  and  the  intima,  formed  of  a  single  row  of  smooth  muscle- 
cells  wound  transversely  or  obliquely  around  the  vessel.  This  layer 
is  called  the  media  or  musculosa,  and  a  vessel  having  these  three  sim- 
ple layers  in  its  walls  is  called  an  arteriole.  In  these  three  layers, 
intima,  media,  and  adventitia,  we  have  the  types  of  all  the  layers  which 
occur  in  the  walls  of  the  largest  and  most  complicated  blood-vessels. 
The  individual  layers  become,  indeed,  more  complex  in  structure ; 
but,  with  the  exception  of  elastic  elements,  no  new  tissues  appear. 
Turning,  now,  to  a  larger  artery — one  of  the  digitals,  for  example — and 
examining  the  various  layers  in  its  walls,  we  find  that  the  intima  is  no 
longer  formed  of  a  simple  endothelial  tube,  but  that  outside  of  this  a 
newr  layer  has  appeared,  composed  of  ill-defined  fibrillated  and  of 
elastic  fibres,  among  which  lie  large,  flattened,  branching  cells.     This 


BLOOD-VESSELS — LYMPHATIC   VESSELS.  61/ 

layer  is  called  the  intermediary  layer  of  the  intima,  and  is  sharply  sep- 
arated from  the  media  by  a  fenestrated,  elastic  membrane,  called  the 
membrana  elastica  intimce. 

The  media  presents  here  quite  a  thick  layer  of  smooth  muscle-cells, 
passing  transversely  around  the  vessel,  and  among  these  we  find  a  few 
elastic  fibres  which  are  connected  with  the  elastic  elements  in  the 
intima  and  adventitia.  The  adventitia  is  thicker,  and  consists  chiefly 
of  fibrillar  connective  tissue  with  elastic  fibres.  In  the  larger  arteries, 
such  as  the  carotids,  aorta,  etc.,  we  find  that  the  individual  layers  are 
considerably  less  sharply  defined  ;  in  the  media  the  elastic  tissue  is 
very  abundant,  taking  the  place,  to  a  considerable  extent,  of  the  mus- 
cular elements ;  it  is  arranged  in  irregularly  distributed  lamellae  and 
fibres,  between  which  lie  fibrillated  fibres  and  connective  tissue  and 
smooth  muscle-cells,  the  latter  no  longer  uniformly  lying  transversely 
to  the  axis  of  the  vessel.  In  the  adventitia,  also,  of  the  large  vessels, 
the  elastic  elements  are  very  numerous,  being  most  abundant  in  the 
vicinity  of  the  media.  In  the  adventitia  of  some  of  the  iarge  vessels, 
smooth  muscle-cells  occur,  arranged  usually  with  their  long  axes 
parallel  with  the  axis  of  the  vessel. 

The  walls  of  the  veins,  like  those  of  the  arteries,  consist  of  three 
layers,  and  these  layers  have  in  general  an  analogous  structure ;  but  they 
are  neither  as  distinct,  nor  are  their  structural  features  as  constant,  as 
those  of  the  arteries.  Moreover,  we  find  that  veins  of  the  same  cali- 
bre present,  in  different  parts  of  the  body,  marked  differences  in  struc- 
ture, and.  unlike  the  arteries,  the  thickness  of  their  walls  is  not  uni- 
formly proportional  to  the  calibre  of  the  vessel. 

In  general  we  may  express  the  structural  difference  between  veins 
and  arteries  by  saying  that  in  the  walls  of  the  former  the  elastic  and 
muscular  elements  are  much  less,  while  the  fibrillar  connective-tissue 
elements  are  more  abundant  than  in  the  walls  of  the  latter.  Now, 
since  the  muscular  and  elastic  elements  are  the  chief  constituents  of 
the  media  and  the  fibrillar  connective-tissue  elements  of  the  adven- 
titia, we  find,  in  general,  that  in  the  veins  the  media  is  less,  while  the 
adventitia  is  more  developed.  The  intermediary  layer  of  the  intima  is 
entirely  absent  in  small  veins;  in  many  of  medium  size  it  appears,  and 
is  again  absent  in  the  largest  vessels.  In  some  veins,  such  as  those  of 
the  bone,  central  nervous  system,  retina,  etc.,  the  muscular  elements 
are  almost  or  entirely  absent.  In  certain  other  veins,  on  the  contrary, 
such  as  the  v.  portarum,  v.  renalis,  the  adventitia  contains  a  great 
abundance  of  muscular  elements  arranged  parallel  with  the  axis  of  the 
vessel. 

The  valves  of  the  veins  consist  of  bundles  of  fibrillar  connective 
tissue  arranged  to  form  a  membranous  projection  from  the  walls  of 
the  vessels — the  bundles  being  arranged,  in  general,  m  a  direction 
parallel  with  the  free  edge  of  the  valve.  They  contain  also  a  net-work 
of  elastic  fibres,  which,  at  that  surface  of  the  valve  which  is  exposed  to 
the  blood-current,  form  a  dense  layer  like  the  intermediary  layer  of 
the  vein-wall  itself.  The  whole  free  surface  is  covered  with  endo- 
thelium like  that  lining  the  general  surface  of  the  vessel. 


NORMAL   HISTOLOGY. 


Endocardium  and  Valves  of  the  Heart. 

The  endocardium,  which  differs  somewhat  in  thickness  and  structure 
in  different  parts  of  the  heart,  consists,  in  general,  of  a  membranous 
expansion  of  fibrillar  connective  tissue,  with  elastic  fibres  and  smooth 
muscle-elements,  which  lines  the  cavities  of  the  heart  and  is  covered 
on  its  free  surface  with  a  layer  of  endothelial  cells.  If  we  examine 
sections  made  perpendicular  to  the  surface  of  the  endocardium,  we 
find  that  just  beneath  the  endothelium  is  a  layer  of  fibrillar  connective 
tissue,  with  flattened  cells,  reinforced  by  a  net-work  of  elastic  fibres, 
which  become  coarser  and  more  abundant  in  that  portion  of  the  layer 
lying  farthest  from  the  heart-cavities.  In  this  layer  are  smooth  muscle- 
cells  running  in  various  directions.  Outside  of  this  layer,  and  joining 
it  to  the  muscular  tissue  proper  of  the  heart,  is  a  layer  of  loose  fibril- 
lar connective  tissue,  in  which  the  blood,  lymphatic  vessels  and  nerves 
lie  embedded.  The  valves  of  the  heart  consist  of  fibrillar  connective 
tissue  arranged  in  membranes  or  fascicles  and  associated  with  elastic 
tissue,  the  latter  being  most  abundant  at  the  free  surfaces  of  the  valves  ; 
and  here  it  is  present  in  the  greatest  quantity  and  density  on  the  sur- 
face which  is  most  directly  exposed  to  the  current  of  blood — that  is, 
on  the  auricular  surfaces  of  the  tricuspid  and  mitral,  and  on  the  ven- 
tricular surfaces  of  the  aortic  and  pulmonary  valves.  The  elastic  ele- 
ments are  also  more  abundant  in  the  valves  of  the  left  than  of  the 
right  side  of  the  heart.  The  firmness  and  capacity  for  resistance  of 
the  elastic  tissue  being  borne  in  mind,  the  significance  of  its  distribu- 
tion in  the  valves  will  be  readily  perceived  :  where  they  are  the  most 
exposed  to  the  impact  and  pressure  of  the  blood,  there  they  are  the 
most  firm  and  dense. 

Lymphatic  Vessels. 

The  larger  lymphatic  vessels  have  a  structure  quite  analogous  with 
that  of  the  veins,  and  like  the  latter  they  are  supplied  with  valves. 
They  approach  the  arterial  type,  however,  in  that  the  muscular  fibres 
are  quite  abundant  in  proportion  to  the  thickness  of  the  walls.  Fol- 
lowing the  larger  lymphatics  toward  the  periphery,  we  find  that  they 
pass  over  into  irregular  branching  and  pouching  channels,  the  walls  of 
which  consist  of  a  single  layer  of  endothelial  cells,  whose  edges  are 
very  sinuous,  dovetailing  into  one  another  like  the  pieces  of  a  child's 
puzzle-map.  These  channels  are  called  lymphatic  capillaries.  Be- 
tween these  and  certain  spaces  or  lacunae  in  the  tissues — those,  for 
example,  in  which  the  connective-tissue  cells  lie  (see  page  26) — there 
is  a  communication,  whose  exact  nature  we  do  not  as  yet  know,  by 
means  of  which  fluids,  and  probably  formed  elements  such  as  blood- 
cells,  pass  over  from  the  blood  into  the  lymphatic  vessels. 

practical  study. 

Capillaries . — The  general  appearance  of  the  capillaries,  as  well  as 
of  the  smaller  arteries  and  veins,  is  best  seen  in  those  parts  in  which 


BLOOD-VESSELS— LYMPHATIC   VESSELS.  69 

the  vessels  are  surrounded  by  but  little  tissue,  as  in  thin  membranes 
such  as  the  mesentery  or  pia  mater.  A  small  bit  from  that  portion  of 
the  pia  which  dips  into  the  sulci  on  the  surface  of  the  hemispheres 
should  be  carefully  separated  from  the  brain  substance,  stretched  on 
a  bit  of  cork  and  fastened  with  pins,  and  laid  for  twenty-four  hours  in 
Miiller's  fluid,  then  for  twenty-four  hours  in  alcohol;  it  is  then  stained 
with  picro-carmine  and  mounted  in  glycerine.  In  such  a  preparation, 
although  we  see  the  elongated  nuclei  in  the  capillary  wall,  we  cannot, 
as  a  rule,  make  out  the  outlines  of  the  cells.  To  accomplish  this  we 
have  recourse  to  the  use  of  dilute  solutions  of  nitrate  of  silver.  This 
can  be  applied  by  immersing  some  thin  membrane,  such  as  the  mesen- 
tery, for  an  hour  in  a  solution  of  nitrate  of  silver  (1-500),  brushing 
off  the  endothelium  from  the  surface,  and  exposing  the  specimen  in 
water  to  the  light  until  it  becomes  brown.  Or,  what  is  better,  the 
entire  vascular  system  of  a  small  animal,  such  as  a  frog,  may  be  first 
rinsed  out  with  water  through  a  canula  introduced  into  the  aorta  and 
attached  to  a  syringe,  and  then  injected  with  the  above  silver  solution. 
Thin  membranes,  such  as  the  mesentery  or  bladder,  are  removed  from 
the  animal,  exposed  to  the  light  for  a  sufficient  time,  and  then,  either 
stained  or  unstained,  mounted  in  glycerine. 

Arteries  and  Veins. — Very  small  vessels  can  be  studied  entire, 
since  by  careful  focussing  we  can  bring  one  portion  after  another  into 
view,  obtaining  thus  what  are  called  optical  sections.  In  the  larger 
vessels,  however,  this  simple  method  is  no  longer  practicable ;  hence, 
we  have  to  resort  to  actual  sections  of  the  vessels. 

Small  arteries  and  veins  may  be  prepared  for  cutting  by  simply 
stretching  them  gently,  when  quite  fresh,  over  a  bit  of  wood  with  pins, 
and  allowing  them  to  dry.  The  dried  vessels  are  placed  between 
two  bits  of  soft  cork,  and  thin  sections  cut  in  any  desired  direction 
with  a  sharp,  hard-tempered,  dry  razor.  The  sections  swell  out  on 
being  dropped  into  water ;  they  are  stained  with  hsematoxylin  and 
mounted  in  glycerine.  Larger  vessels,  such  as  the  aorta,  vena  cava, 
etc.,  may  be  simply  hardened  in  alcohol  embedded  in  hardened  liver 
and  cut;  then  stained  with  haematoxylin  and  mounted  in  glycerine 
or  balsam. 

Heart-valves  a?id  Endocardium. — These  may  be  studied  in  thin 
sections  made  perpendicular  to  the  surface  of  the  valves  and  the  inner 
surface  of  pieces  of  the  heart  which  have  been  stretched  on  cork  and 
hardened  in  alcohol. 

Lymphatic  Vessels. — A  general  view  of  the  relation  between  the 
larger  lymphatic  trunks  and  the  lymphatic  capillaries  is  best  obtained 
by  studying  nitrate  of  silver  preparations  of  some  part  in  which  the 
latter  are  abundant.  Such  a  structure  we  find  in  the  tendinous  por- 
tion of  the  diaphragm — so-called  central  tendon — of  the  rabbit,  which 
is  very  thin,  covered  on  both  sides  with  endothelium  and  very  richly 
supplied  with  lymphatic  vessels. 

To  prepare  the  central  tendon,  a  rabbit  should  be  killed  by  break- 
ing up  the  medulla  with  a  wire,  and  the  thoracic  and  abdominal  cavities 
freely  opened  by  incisions  above  and  below  the  anterior  line  of  attach- 
ment of  the  diaphragm.     Two  ligatures  are  passed  around  the  large 


70  NORMAL   HISTOLOGY. 

vessels  above  the  diaphragm,  to  prevent  the  escape  of  blood  on  to  the 
part,  the  vessels  cut  across  between  them,  and  the  heart  and  lungs 
removed  from  the  chest.  The  suspensory  ligament  of  the  liver  is  now 
severed,  and  the  diaphragm  is  left  free,  stretching  across  between  the 
thoracic  and  abdominal  cavities.  Its  upper  surface  is  brushed  with  a 
soft  pencil  moistened  with  water,  so  as  to  remove  the  surface  endothe- 
lium and  allow  the  silver  to  penetrate  readily  into  the  lymphatics. 
The  surface  is  now  carefully  washed  and  a  one-half  per  cent,  solution 
of  nitrate  of  silver  is  allowed  to  run  over  it  and  remain  for  five  minutes 
in  contact  with  it.  This  is  now  washed  off  with  water,  the  diaphragm 
carefully  cut  out  and  pinned  to  a  cork,  so  as  to  lie  flat,  and  exposed 
to  the  light  in  dilute  alcohol  till  it  becomes  brown.  Small  pieces  are 
then  cut  out  from  the  tendinous  portion  and  mounted  in  glycerine.  In 
such  a  specimen  the  large  lymphatic  channels  will  be  readily  recog- 
nized lined  with  elongated  endothelium  like  that  of  the  blood-vessels, 
and  furnished  with  valves,  which  appear  here  and  there  as  curved 
lines  crossing  the  vessel.  The  capillaries  may  be  recognized  by  their 
irregular  shape  and  sinuous  endothelium. 

In  the  basement  substance  which  lies  between  the  lymph-ves- 
sels and  is  stained  brown,  variously  shaped  larger  and  smaller  irregular 
light  spaces  are  seen,  sometimes  apparently  communicating  with  one 
another  and  with  the  lymphatic  vessels.  These  irregular  spaces  in  the 
basement  substance  are  variously  interpreted  ;  some  regarding  them 
as  the  ultimate  radicles  of  the  lymphatic  vessels  and  terming  them 
serous  canaliculi ;  others  thinking  that  they  are  simply  spaces  in  the 
connective  tissue  bearing  the  same  relation  to  the  lymph-vessels  that 
the  connective-tissue  spaces  elsewhere  in  the  body  do. 


CHAPTER  IX. 

THE    GASTRO-INTESTINAL    CANAL. 

This  canal  is  a  tube  varying  greatly  in  its  calibre  in  different  parts, 
and  continuous  at  either  end  with  the  external  surface  of  the  body. 
In  certain  parts  of  its  course  it  is  intimately  connected  with  adjacent 
structures ;  but,  for  the  most  part,  it  is  attached  only  at  one  side  by  a 
structure  which  serves  to  convey  to  it  its  blood  and  lymphatic  vessels 
and  nerves.  The  walls  of  the  tube,  although  varying  in  structure  in 
different  sections,  consists  in  general  of  a  muscular  layer,  a  mucous 
layer  lining  the  tube,  and  in  those  parts  where  it  is  suspended  in  the 
abdominal  cavity  a  serous  layer  covering  it.  Confining  our  attention 
to  the  stomach  and  intestines,  we  find  that  these  layers  are  not  simple, 
but  have  each  a  composite  structure  ;  thus,  we  find  in  the  serosa  a 
layer  consisting  chiefly  of  dense  fibrillar  connective  tissue,  sud-serosa, 
covered  with  a  layer  of  endothelium.  The  muscular  tunic,  or  muscu- 
losa,  consists  of  two  layers  of  smooth  muscular  tissue :  an  external,  in 
which  the  cells  lie  longitudinally,  and  an  internal,  in  which  they  lie 
transversely  to  the  axis  of  the  canal.  In  certain  parts  of  the  stomach 
an  indistinct  third  layer  is  found,  in  which  the  cells  have  an  oblique 
course.  Inside  the  muscularis  and  joining  it  to  the  mucosa,  is  a  layer 
of  loose  fibrillar  connective  tissue,  in  which  the  blood  and  lymphatic 
vessels  ramify,  called  the  submncosa.  In  the  mucosa,  finally,  we  have 
a  delicate  supporting  framework  of  connective  tissue  varying  some- 
what in  its  structure  and  abundance  in  different  parts  of  the  canal ; 
this  is  covered  by  epithelial  cells  and  contains  the  glandular  apparatus, 
while  at  the  base  of  the  glands  and  adjacent  to  the  submucosa  is  a 
thin  layer  of  smooth  .muscle-cells,  lying  in  both  transverse  and  oblique 
directions,  called  the  muscularis  mucosa,  from  which  usually  a  few 
muscle-cells  pass  up  between  the  glands. 

The  chief  differences  in  minute  structure  between  the  stomach  and 
intestines  are  in  the  mucous  membrane,  and  since  in  these  membranes 
the  glands  are  very  important  factors,  a  word  should  be  said  here  about 
the  structure  of  glands  in  general.  Although  the  term  gland  is  popu- 
larly applied  to  structures  having  the  greatest  diversity  of  form  and 
function,  and  little  in  common  but  their  name,  we  mean  by  it  here 
those  organs  whose  physiological  activity  expresses  itself,  in  part  at 
least,  by  the  elaboration  of  certain  specific  fluids,  secretions,  or  excre- 
tions. All  such  glands  have  a  somewhat  analogous  structure  and 
present  two  distinct  kinds  of  structural  elements  :  i.  Epithelial  or 
gland  cells ;    2.  A  connective-tissue  framework,  with  blood  and  lym- 


72  NORMAL   HISTOLOGY. 

phatic  vessels  and  nerves.  The  epithelial  cells,  usually  large,  differ 
in  form  in  different  and  even  in  the  same  gland ;  in  the  latter  case 
especially,  when  the  gland  is  divided  into  a  secreting  and  excretory 
portion.  They  will  be  described  when  we  study  the  glands  in  detail. 
The  connective  tissue,  varying  greatly  in  amount  in  different  glands, 
is  sometimes  arranged  in  sheets  and  bundles  so  as  to  form  variously 
shaped  cavities  which  are  lined  with  the  gland-epithelium  ;  sometimes, 
in  the  form  of  simple  or  superimposed  lamellae  covered  with  flat  cells 
like  endothelium,  it  forms  thin-walled  tubes  or  chambers  on  whose 
sides  the  cells  are  placed ;  in  this  form  it  is  called  the  membrana 
propria  of  the  gland-cavities. 

The  cavities  and  tubes  thus  formed  and  lined  with  gland-epithelium 
are  variously  arranged  in  different  glands,  but  their  different  modes  of 
arrangement  may  be  reduced  to  three  types  : 

i.  Tubular  glands,  which  have  the  form  of  simple  or  occasionally 
branching,  straight,  curved  or  variously  contorted  tubes,  terminating  in 
blind  extremities.     Such  are  the  glands  of  the  stomach. 

2.  Racemose  glands,  in  which  the  secreting  portion  in  the  form  of 
vesicular  or  irregular-shaped  cavities — alveoli — are  grouped  around 
simple  or  branching  excretory  ducts  into  which  they  open ;  the  whole 
structure  has  been  not  inaptly  compared  to  a  bunch  of  grapes,  in  which 
the  fruit  would  correspond  to  the  alveoli,  the  stem  to  the  excretory 
ducts  ;  the  analogy  failing  at  this  point,  however,  for  in  the  gland  the 
alveoli  and  ducts  are  bound  together  by  connective  tissue  lying  between 
them — called  interstitial  tissue — in  which  the  vessels  and  nerves  ramify. 
Such  are  the  mammary  glands  and  certain  mucous  glands  of  the 
bronchi. 

3.  Vesicular  Glands. — These  consist  of  simple,  spheroidal,  or  irre- 
gular-shaped closed  alveoli,  surrounded  by  a  membrana  propria  and 
lmed  with  epithelium,  the  separate  alveoli  being  embedded  in  inter- 
stitial connective  tissue.     Such  glands  are  the  thyroid  and  ovary. 

The  Stomach. 

The  muscularis  of  the  stomach  differs  from  that  of  the  intes- 
tines, in  that  a  certain  number  of  the  cells,  especially  in  the  cardiac 
extremity,  do  not  have  the  typical  transverse  or  longitudinal  arrange- 
ment, but  lie  in  an  oblique  direction.  In  the  vicinity  of  the  pylorus 
again,  the  inner  circular  layers  are  much  thickened,  forming  the 
sphincter  pylori.  The  mucosa  is  almost  entirely  made  up  of  glands 
supported  and  held  together  by  a  small  amount  of  delicate  connective 
tissue,  in  which  the  blood  and  lymphatic  vessels  ramify.  The  glands 
are  tubular,  usually  simple,  but  sometimes  divided  at  the  base.  They 
have  a  membrana  propria,  and,  depending  upon  differences  in  the 
epithelium  which  line  them,  they  are  classified  as  :  1.  Mucous  glands  ; 
2.  Peptic  glands.  The  general  surface  of  the  stomach  is  covered  with 
cylindrical  epithelium.  In  the  pyloric  region,  and  here  and  there  in 
other  parts,  the  glands,  or  follicles  as  they  are  often  called,  are  lined 
throughout  with  cylindrical  epithelium ;  these  are  the  mucous  glands. 

The  greater  proportion  of  the  glands,  however,  are  lined  only  at 


THE   GASTROINTESTINAL   CANAL.  73 

their  orifices  with  cylindrical  epithelium  ;  deeper  down  in  the  gland 
we  find  usually  two  kinds  of  cells  :  a,  rounded  or  cuboidal  cells  with 
transparent  or  very  finely  granular  bodies  ;  and  b,  larger  spheroidal  or 
somewhat  flattened  very  granular  cells,  which  usually  lie  outside  the 
others,  between  them  and  the  membrana  propria.  These  are  the  so- 
called  peptic  cells,  and  these  glands  are  called  peptic  glands. 

The  relative  number  of  these  different  kinds  of  peptic  cells  varies, 
depending  upon  the  degree  of  functional  activity  of  the  glands.  When 
they  are  secreting  rapidly,  the  granular  cells  are  abundant ;  when  at  rest 
they  are  few  in  number,  the  smaller  transparent  cells  preponderating. 

The  arteries  pass  obliquely  through  the  serosa  and  musculosa, 
divide  and  subdivide  in  the  loose  tissue  of  the  submucosa,  from  whence 
branches  are  sent  in  between  the  glands ;  here,  before  reaching  the 
surface,  they  break  up  into  a  close  capillary  net  surrounding  the  folli- 
cles, and  the  blood  is  finally  collected  into  narrow  venous  trunks 
directly  beneath  the  surface  epithelium  ;  from  these  it  passes  back 
into  larger  veins  in  the  submucosa,  where  it  collects  in  the  efferent 
veins.  The  lymphatic  vessels  lie  between  the  glands,  form  anastomos- 
ing channels  in  the  submucosa,  and  pass  out  through  the  musculosa, 
receiving  larger  and  smaller  trunks  from  the  latter.  In  the  connective- 
tissue  framework  of  the  mucosa,  between  the  follicles — sometimes  at 
their  base,  sometimes  between  them  nearer  the  surface — are  found  cir- 
cumscribed collections  of  small  round  cells,  apparently  analogous  with 
the  lymphatic  follicles  presently  to  be  described  in  the  intestines. 
These  in  man  are  variable  in  their  occurrence,  being  sometimes  abun- 
dant, sometimes  not ;  they  are  apt  to  be  most  abundant  in  the  pyloric 
region,  and  are  often  visible  as  small  grayish  prominences  on  the 
surface  of  the  mucous  membrane.  They  are  frequently  called  the 
lenticular  glands  of  the  stomach. 

The  Small  Intestine. 

The  supporting  connective-tissue  frame-work  of  the  mucosa  in 
the  small  intestine  is  more  abundant  than  in  the  stomach,  and  is 
richly  infiltrated  with  small  spheroidal  and  variously  shaped  cells. 
In  it  lie  embedded  tubular  glands  not  unlike  the  mucous  glands  of 
the  stomach,  but  not  crowded  so  closely  together.  These  glands 
are  often  called  the  follicles  of  Lieberkuhn,  and  are  lined  with  cylin- 
drical epithelium.  Rising  from  the  general  surface  of  the  mucous 
membrane,  between  the  orifices  of  the  glands,  are  very  numerous 
short,  cylindrical  or  conical  projections  called  villi.  These  are  formed 
by  projections  inward  of  the  mucosa;  they  are  covered  by  cylin- 
drical epithelium,  and  contain  an  abundant  vascular  net-work  and  the 
radicles  of  the  lymph-  or  chyle-vessels.  The  cylindrical  epithelium 
possesses  a  marked  peculiarity  in  the  structure  of  its  free  border,  which 
is  considerably  thickened,  and  is  crossed,  in  a  direction  corresponding 
with  the  long  axis  of  the  cell,  by  fine  parallel,  closely-set  lines,  which 
are  usually  interpreted  as  tiny  pores  or  canals  passing  through  the 
border.  Scattered  here  and  there  between  the  cylindrical  epithelium, 
sometimes  abundant,  sometimes  not,  their  number  often  apparently 


74  NORMAL   HISTOLOGY. 

depending  in  a  measure  upon  the  preservative  fluid  employed,  clear 
more  or  less  ovoidal  cells  are  seen,  with  a  nucleus  and  a  small  amount 
of  protoplasm  in  the  vicinity  of  the  narrow  base  ;  they  look  as  if  the 
free  border  of  the  cell  had  fallen  off  and  most  of  the  cell-contents  had 
disappeared.  Not  infrequently  a  translucent  structureless  substance 
is  seen  protruding  from  the  open  end  of  the  cell,  as  if  in  the  act  of 
passing  out  of  it.  These  cells  are  called,  from  their  form,  goblet-cells. 
Their  significance  is  not  yet  definitely  determined  in  ali  cases  :  by 
some  they  are  regarded  as  cylindrical  cells  changed  by  artificial  means  ; 
others  believe  them  to  be  normal  structures ;  by  many  it  is  supposed 
that  under  certain  circumstances  the  cell-contents  undergo  a  mucous 
metamorphosis,  swell  up,  burst  out  of  the  cell,  leaving  little  but  the 
membrane  and  nucleus  behind,  and  that  thus,  under  normal  conditions, 
a  certain  amount  of  the  mucus  furnished  by  mucous  membranes  is  pro- 
duced. In  addition  to  the  tubular  glands  which  are  found  throughout 
the  whole  extent  of  the  small  intestine — in  the  duodenum,  especially 
in  its  upper  portions — racemose,  probably  mucous  glands,  are  found, 
called  Brunner's  glands.  They  lie  in  the  submucosa,  and  consist  of 
variously  shaped,  but  usually  follicular  alveoli  surrounded  by  a  mem- 
brana  propria  and  lined  with  cylindrical  epithelium.  The  excretory 
ducts  are  also  lined  with  cylindrical  epithelium,  and  open  on  the  surface 
of  the  mucous  membrane.  Smooth  muscle-cells  pass  up  from  the 
muscularis  mucosae  into  the  villi.  The  central  portion  of  the  villi  is  oc- 
cupied by  one,  or  more,  usually  blind  canals — the  chyle-vessels — which 
pass  outward  to  the  bases  of  the  villi,  where  they  usually  unite  to  form 
a  net-work  around  the  orifices  of  the  tubular  glands  of  Lieberkiihn  and 
the  lymph-follicles  presently  to  be  described;  they  then  pass  into 
the  submucosa,  where  they  form  larger  anastomosing  channels  ;  from 
thence  trunks  pass  through  the  musculosa,  receiving  vessels  from  its  two 
layers  and  from  a  well-developed  net-work  between  them. 

Closely  connected  with  the  lymphatic  vessels,  and  apparently  form- 
ing a  part  of  the  lymphatic  apparatus  of  the  intestines,  are  found  cer- 
tain structures  called  lymphatic  follicles,  and  of  these  it  is  customary  to 
distinguish  two  kinds  :  i.  Solitary  follicles ;  and  2.  Agminated  folli- 
cles, or  Peyer's  patches. 

1.  Solitary  Follicles. — These  are  irregularly  scattered  through  the 
mucous  membrane  of  both  small  and  large  intestines,  in  the  form  of 
small  grayish  nodules.  They  lie  chiefly  in  the  mucosa,  often  piercing 
the  muscularis  mucosce  and  desce?iding  into  the  submucosa;  they  are 
usually  spheroidal  or  pear-shaped,  and  frequently  project  somewhat  into 
the  intestinal  cavity.  Where  they  lie,  the  tubular  glands  are  crowded 
to  one  side  and  the  villi  are  absent  over  their  surfaces.  They  may  lie 
so  near  the  surface  as  to  be  covered  only  by  a  single  layer  of  cylindri- 
cal epithelium,  or  they  may  be  more  deeply  placed,  and  covered  in 
addition  by  a  thin  layer  of  the  connective  tissue  of  the  mucosa.  They 
consist  of  a  nodule  of  reticular  connective  tissue,  whose  meshes  are 
somewhat  narrower  at  the  periphery  where  it  becomes  continuous 
with  adjacent  parts.  The  meshes  are  closely  filled  with  small  sphe- 
roidal cells,  having  the  characters  of  lymph-cells.  In  their  periphery 
the  lymph-vessels  of  the  mucous  membrane  form  a  closely  anasto- 


THE   GASTROINTESTINAL   CANAL.  75 

mosing  net-work.  The  blood-vessels  also  interlace  in  their  periphery, 
and  send  an  abundance  of  anastomosing  capillary  loops  into  their  in- 
terior. 

2.  Peyer's  Patches. — These  are  found  chiefly  in  the  small  intes- 
tine, and  here  are  most  abundant  in  the  lower  portion  of  the  jejunum 
and  in  the  ileum  ;  they  are  round,  or  more  frequently  elongated,  usually 
slightly  elevated  structures,  and  are  always  situated  at  the  side  oppo- 
site the  mesenteric  attachment,  with  their  long  axis  parallel  with  the 
axis  of  the  gut.  They  consist,  essentially,  of  an  aggregation  of  a  va- 
riable number  of  structures,  having  the  characters  of  the  solitary  folli- 
cles ;  these  are  placed  closely  together,  side  by  side,  and  supplied  in 
essentially  the  same  way  as  the  solitary  follicles  with  blood  and  lym- 
phatic vessels. 

The  Large  Intestine. 

The  mucosa  of  the  large  intestine  is  thickly  set  with  tubular  glands 
similar  to  Lieberkuhn's  glands  in  the  small  intestine,  but  is  destitute 
of  villi.  Solitary  lymphatic  follicles  are  abundant  and  are,  as  a  rule, 
somewhat  larger  than  those  of  the  small  intestine.  The  distribution 
of  blood  and  lymphatic  vessels  resembles  in  most  respects  that  de- 
scribed in  the  stomach. 

PRACTICAL    STUDY. 

Stomach. — Bits  of  perfectly  fresh  rabbit's  or  dog's  stomach,  from 
the  fundus  and  the  pyloric  region,  should  be  stretched  on  a  bit  of  cork 
to  prevent  shrinkage,  and  immersed  in  absolute  alcohol.  After  twen- 
ty-four hours  the  fluid  should  be  changed,  and  in  three  or  four  days  the 
specimen  will  probably  be  hard  enough  to  cut.  Sections  from  both 
regions  are  made  perpendicular  to  the  surface  after  embedding  in  wax. 
They  are  stained  with  hematoxylin  and  eosin,  and  mounted  in  glyce- 
rine or  Canada  balsam.  The  nuclei  of  all  the  cells  are  stained  violet 
by  the  hematoxylin ;  in  the  peptic  glands  the  bodies  of  the  granular 
peptic  cells  are  stained  a  deep  rose-red,  while  the  others  are  but 
slightly  colored,  or  not  at  all.  Sections  perpendicular  to  the  surface 
of  a  stomach,  whose  blood-vessels  are  filled  with  the  mixture  of  Prus- 
sian-blue and  gelatine  and  stained  with  carmine,  are  very  instructive. 

Intestine. — Bits  of  intestine  from  the  upper  portion  of  the  duo- 
denum, from  the  ileum,  including  one  of  Peyers  patches,  and  from 
the  large  intestine  should  be  stretched  on  cork  and  immersed  in  a 
mixture  of  equal  parts  of  half  per  cent,  chromic  acid  and  alcohol ; 
after  four  days,  transferred  to  alcohol,  in  which,  in  a  day  or  two,  it 
will  become  hard  enough  to  cut.  Perpendicular  sections  from  the 
various  parts  should  be  made  and  stained,  and  mounted  in  the  same, 
way  as  the  stomach.  Sections  should  be  made  from  intestines  whose 
blood-vessels  are  injected  with  blue,  stained  with  carmine,  and  mounted 
in  Canada  balsam. 


CHAPTER  X. 

SUBMAXILLARY    GLAND— LIVER. 

Submaxillary  Gland. 

Connected  with  the  digestive  tract  are  several  racemose  glands, 
which,  although  differing  in  important  particulars,  both  in  structure 
and  function,  yet  have  many  features  in  common.  These  glands  are 
the  submaxillaris,  the  sublingual,  the  parotid,  and  the  pancreas. 
The  details  of  structure  of  these  glands  are  still  insufficiently  known, 
and  within  the  limits  of  these  lessons  we  cannot  consider  at  length  even 
what  is  well  understood.  We  will  simply  look  at  some  of  the  more 
important  features  of  one  of  the  best  known,  the  submaxillaris,  con- 
sidering this,  in  a  general  way  only,  as  typical  of  the  others. 

The  submaxillary  gland  differs  in  structure  in  different  animals,  its 
structure  in  the  dog  being,  perhaps,  best  known,  and  quite  closely  re- 
sembling that  in  man.  In  the  dog  it  consists,  like  other  racemose 
glands,  of  alveoli  and  excretory  ducts.  The  alveoli  are  surrounded 
by  a  rnembrana  propria,  and  grouped  into  lobules  by  more  or  less  in- 
terstitial connective  tissue,  which  is  furnished  with  blood-  and  lymph- 
vessels  and  nerves.  The  alveoli,  depending  upon  the  gland-epithe- 
lium which  lines  them,  present  two  distinct  forms  :  in  one  form,  the 
smaller  of  the  two,  we  find  the  cavity  of  the  alveoli  nearly  filled  by 
cuboidal  or  polyhedral  cells,  whose  bodies  are  cloudy  or  granular,  and 
which  have  spheroidal  or  ellipsoidal  nuclei ;  these  cells  are  readily 
stained  with  carmine.  In  the  other  form  of  alveoli — the  larger — we  find 
in  the  first  place,  surrounding  the  cavity  of  the  alveoli,  large,  irregular- 
shaped,  transparent  cells,  having  a  gelatinoid  appearance,  with  an  often 
flattened  nucleus  lying  at  the  peripheral  side  ;  these  cells  are  little, 
if  at  all,  stained  by  carmine.  In  the  second  place,  in  the  periphery  of 
the  alveoli,  between  the  cells  just  described  and  the  rnembrana  pro- 
pria, lie  large,  in  cross-section,  crescentic,  strongly  granular  masses, 
usually  containing  several  nuclei.  These  are  believed  to  be  formed 
by  a  number  of  small,  angular  cells,  closely  crowded  together.  They 
are  readily  stained  with  carmine,  and  are,  apparently,  the  analogues 
of  the  granular  peptic  cells  of  the  stomach.  Like  the  latter  they  are 
most  abundant  when  the  gland  is  in  a  condition  of  functional  activity, 
and  are  supposed  to  be  destined  to  replace  the  inner  layer  of  trans- 
parent cells,  as  these  are  destroyed  in  furnishing  the  specific  secretion 
of  the  gland.  The  excretory  ducts  differ  in  their  structure  in  differ- 
ent parts  of  their  course.  The  portion  which  lies  outside  of  the  gland 
proper,  conveying  its  secretion  to  the  mouth,  is  formed  by  a  tube  of 


SUBMAXILLARY   GLAND— LIVER.  JJ 

connective  tissue,  supported  by  elastic  fibres,  and  lined  by  a  single 
layer  of  cylindrical  epithelium.  Inside  the  gland  the  ducts  present 
two  types  of  structure  :  first,  we  find  passing  out  from  the  alveoli, 
broad  tubes  formed  of  a  membrana  propria  similar  to  that  of  the  alve- 
oli, and  lined  by  cuboidal  or  flattened,  somewhat  granular  cells  ; 
second,  joining  the  last  described  ducts  to  those  lying  outside  the  sub- 
stance of  the  gland  proper  are  tubes  formed  of  essentially  the  same 
membrana  propria,  but  lined  with  cylindrical  nucleated  cells,  whose 
bodies,  in  the  peripheral  zone,  are  distinctly  striated  in  a  direction 
parallel  with  the  axis  of  the  cell,  and  in  the  central  zone,  bordering  on 
the  cavity  of  the  tube,  finely  granular. 


PRACTICAL    STUDY. 

Sections  of  Gland  from  Dog. — Small  pieces  of  a  perfectly  fresh 
gland  are  put  into  absolute  alcohol,  and,  when  sufficiently  hardened, 
thin  sections  are  made,  stained  with  hematoxylin  and  eosin,  and 
mounted  in  Canada  balsam. 

Teased  Gland. — To  study  the  minute  structure  of  the  cells,  which 
are  somewhat  altered  by  the  above  method,  small  bits  of  a  fresh  gland 
are  laid  for  twenty-four  hours  in  a  small  quantity  of  one  per  cent, 
osmic  acid  solution,  washed,  teased  apart  on  a  slide  in  glycerine,  and 
stained  with  carmine. 

The  Liver. 

The  liver  presents  three  distinct  elements  of  structure  :  i.  The 
cellular  elements,  which  in  form,  function,  and  arrangement  charac- 
terize the  organ,  the  liver-cells,  or  parenchyma.  2.  The  coiinective- 
tissue  frame-zvork,  the  interstitial  tissue,  which  surrounds  the  organ  as 
a  capsule,  and  in  its  interior  supports  the  parenchyma  and  carries  the 
larger  vessels.  3.  The  blood-,  lymph-,  and  gall-vessels.  •  The  liver- 
cells  are  large,  have  the  form  of  irregular,  often  somewhat  elongated 
polyhedra;  they  have  a  granular  body  which  frequently  encloses 
granules  of  pigment  and  larger  or  smaller  droplets  of  fat ;  they  have 
one  or  more  vesicular  nuclei  and  nucleoli.  When  living  they  are  very 
soft,  and  the  isolated  cells  often  show  depressions  on  their  sides 
caused  by  pressure  from  adjacent  blood-vessels.  If  we  look  at  the 
cut  surface  of  a  fresh  liver  with  the  naked  eye,  we  find  that  it  presents, 
more  or  less  distinctly,  small  polygonal  or  irregular-shaped  figures, 
which  are  sections  of  certain  groups  of  liver-cells,  called  acini  or 
lobules.  These  lobules  have,  in  general,  the  form  of  oblong  polyhe- 
dra,  and  the  difference  in  shape  presented  by  the  sections  is  due  to 
the  fact  that  they  lie  crowded  together,  with  their  long  axes  lying  in 
various  directions.  In  order  to  understand  the  structure  of  these 
lobules,  it  is  necessary  to  study  them  in  connection  with  the  blood- 
vessels of  the  liver,  to  which  they  bear  a  very  constant  and  character- 
istic relation.  The  liver  receives  its  blood  from  the  portal  vein  and 
the  hepatic  artery  ;  it  is  conveyed  away  by  the  hepatic  vein.  If  we 
follow  the  ramifications  of  the  hepatic  vein,  we  find  that  it  divides  and 


78  NORMAL   HISTOLOGY. 

subdivides  until   it   finally  breaks  up    into    short    terminal   radicles, 
around  which  as  a  centre  the  oblong  liver-lobules  are  grouped.     From 
its  smaller  branches  also,  before  it  breaks  up  into  terminal  radicles, 
small,  short  branchlets  are  given  off,  which  form  the  centres  of  lobules. 
Veins  bearing  this  relation  to  the  lobules  are  called  central  veins,  or 
venae  intralobular  es.     If  now  we  follow  the  ramifications  of  the  portal 
vein,  on  the  other  hand,  we  find  that  dividing,  and  subdividing  it,  too, 
gives  off  small  branchlets  which,  with  the  terminal  branchlets  into 
which  it.  finally  breaks  up,  pass  to  the  surface  of  the  lobules ;  there 
they  pour  their  blood  into  a  rich  capillary  net-work  within  the  lobules, 
whence  it  passes  directly  into  the  radicles  of  the  hepatic  vein  at  the 
centre.     The  capillaries  of  the  lobules  radiate  from  the  central  vein, 
for  the  most  part  nearly  at  right  angles  to  its  axis,  and  take  more  or 
less  direct,  slightly  divergent,  courses  to  the  periphery,  being  con- 
nected with  each  other  by  frequent  branches.     A  net-work  is  thus 
formed,  in  whose  narrow  and   elongated    meshes  the  liver-cells  lie, 
sometimes  in  a  single  row,  sometimes  in  several  rows,  depending  upon 
the  breadth  of  the  intercapillary  spaces.     The  central  veins  do  not 
usually  extend  quite   to  the  extremity  of  the  lobules,  and  here  the 
capillaries  are  given  off  brush-like  obliquely  from  its  end.     A  liver- 
lobule,  then,  is  a  circumscribed  portion  of  liver-tissue,  having  for  its 
centre   a  branchlet  of  the  hepatic  vein — vena  intralobular  is,  and  at 
its  periphery  the  terminal  branchlets  of  the  portal  vein — venae  i?iter- 
lobular  es,  while  between  these  two  sets  of  vessels,  and  joining  them, 
is  a  rich  capillary  net-work,  in  whose  elongated  meshes  lie  rows  of 
liver-cells.     In  certain  animals,  such  as  the  pig,  the  lobules  are  very 
distinct,  being  surrounded  by  connective  tissue  which  is  directly  con- 
tinuous with  the  connective  tissue  surrounding  the  larger  trunks  of  the 
portal  vein,  hepatic  artery,  etc.,  and  called  Glisson's  capsule.     In  this 
connective  tissue  surrounding  the  lobules  are  found,  in  addition  to  the 
branchlets  of  the  portal  vein,  certain  of  the  terminal  branchlets  of  the 
hepatic  artery  and    the  capillaries    connected  with    them,   and    the 
smaller  gall-ducts.     In  the  human  liver,  on  the  contrary,  there  is  very 
little  connective  tissue  between  the  lobules,  only  here  and  there  small 
masses  are  seen  surrounding  the  branches  of  the  portal  vein  and  its 
accompanying  vessels,  the  lobules  merging,  for  the  most  part,  into  one 
another.     The  hepatic  artery  sends  its  blood  into  capillaries  which 
are  distributed  largely  to  the  walls  of  the  vessels  and  the  connective 
tissue,  and  it  finally  passes  either  into  the  terminal  branchlets  of  the 
portal  vein  at  the  periphery  of  the  lobules,  or  directly  into  the  intra- 
lobular capillaries. 

We  have  finally  to  consider  the  gall-passages.  The  larger  gall- 
ducts,  lined  with  a  well-developed  mucous  membrane,  supplied  with 
tubular  and  racemose  mucous  glands,  and  covered  with  cylindrical 
epithelium,  enter  the  liver  with  the  other  large  vessels,  and  dividing 
and  subdividing  accompany  them  in  the  capsule  of  Glisson. 

As  they  pass  inward  they  become  smaller  and  smaller,  the  mucous 
membrane  loses  its  glands  and  becomes  simpler  in  structure — the 
small  ducts  consisting  of  little  more  than  a  simple  tube  lined  with  low 
cylindrical   or  cuboidal    cells.     Finally,   as    the   ducts   arrive   at   the 


SUBMAXILLARY   GLAND — LIVER.  79 

periphery  of  the  lobules — intralobular  ducts — they  are  lined  with  flat, 
polygonal  cells  ;  here  they  become  continuous  with  the  intralobular 
gall-passages  or  gall-capillaries.  The  gall-capillaries  are  extremely 
small  and  difficult  of  demonstration  ;  they  form  a  delicate  net-work 
around  the  individual  liver-cells,  being  arranged  in  such  a  way,  how- 
ever, that  they  never  come  into  contact  with  the  blood-capillaries, 
always  being  separated  by  at  least  a  part  of  the  diameter  of  a  liver- 
cell  from  the  latter.  They  do  not  seem  to  possess  a  distinct  wall,  but 
are  rather  simple  channels  grooved  in  the  walls  of  contiguous  liver- 
cells. 

The  connective  tissue  of  the  human -liver  is  chiefly  found  in  the 
capsule  which  surrounds  the  organ  and  in  the  capsule  of  Glisson  ; 
but,  in  addition  to  this,  we  find  it  in  very  small  quantity,  not  only 
in  the  vicinity  of  the  hepatic  vein,  but  also  between  the  cells  and 
along  the  capillaries  within  the  lobules  ;  in  the  latter  situation  it  occurs 
in  the  form  of  delicate  fibres  or  membranes,  with  here  and  there  fusi- 
form or  stellate  cells.  Bearing  in  mind  that  the  lobules,  although  in 
general  of  an  oblong  polyhedral  form,  lie  crowded  together,  more  or 
less  distorted  by  pressure,  with  their  long  axes  running  in  various 
directions  ;  and  further,  that  where  two  or  three  terminal  branchlets  of 
the  hepatic  vein  are  given  off  together,  compound  lobules  may  be 
formed  around  them,  having  a  common  base  with  separate  apices — it 
will  be  readily  conceived  that  sections  through  the  liver  in  any  direc- 
tion will  present  lobules  of  the  most  diverse  form.  But  if  the  charac- 
ter of  the  vessels  at  the  centre  and  periphery  be  borne  in  mind,  the 
determination  of  the  relative  position  of  any  given  portion  will  not,  as 
a  rule,  be  difficult.  The  lymphatic  vessels  of  the  liver  form  an  abun- 
dant net-work  in  the  capsule,  and  also  accompany  the  larger  vascular 
trunks  in  Glisson's  capsule  and  between  the  lobules.  Their  presence 
within  the  lobules  has  not  yet  been  definitely  demonstrated. 

PRACTICAL    STUDY. 

Liver-cells. — A  small  bit  of  a  perfectly  fresh  liver  is  teased  on  a 
slide  in  a  drop  of  one-half  per  cent,  salt  solution,  and  covered  and 
studied  in  the  same  fluid.  If  it  be  desired  to  preserve  the  isolated 
liver-cells,  a  small  bit  of  fresh  liver  is  put  for  twenty-four  hours  in  a 
mixture  of  alcohol  and  water  (i  to  2),  and  then  teased  on  a  slide  in  a 
drop  of  glycerine  which  has  been  slightly  colored  with  eosin,  and 
mounted  in  the  same. 

Sections  of  Pig 's  Liver. — A  small  piece  of  perfectly  fresh  pig's  liver 
is  immersed  for  three  weeks  in  Muller's  fluid,  which  should  be  changed 
once  or  twice,  then  washed  and  transferred  to  alcohol.  When  it  is 
sufficiently  hard  very  thin  sections  should  be  made  near  the  surface, 
both  parallel  and  at  right  angles  to  it,  so  as  to  cut  the  lobules,  which 
are  quite  regularly  arranged  at  the  surface,  in  different  directions. 
The  sections  are  stained  with  hematoxylin  and  mounted  in  Canada 
balsam.  The  lobules  being  here  surrounded  by  tolerably  distinct  lay- 
ers of  connective  tissue,  in  which  the  intralobular  vessels  run,  the 
lobular  structure  is  quite  evident,  and  the  different  pictures  obtained 


SO  NORMAL   HISTOLOGY. 

by  sections  through  the  lobules  in  different  directions  are  not  difficult 
of  interpretation. 

Sections  of  Human  Liver. — Sections  are  made  from  a  bit  of  human 
liver  hardened  as  above,  and  stained  with  hematoxylin  and  eosin,  and 
mounted  in  glycerine.  Here  the  lobular  structure  is  very  ill-defined, 
because  of  the  small  amount  of  connective  tissue  between  the  lobules, 
and  for  the  recognition  of  the  different  parts  of  the  latter  we  are  largely 
dependent  upon  the  determination  of  the  different  kinds  of  blood-ves- 
sels, since  these  always  bear  a  definite  relation  to  the  lobules.  It  is 
to  be  remembered  that  the  branches  of  the  portal  vein  lie  only  in  the 
periphery  of  the  lobules,  that  they  are  usually  accompanied  by  other 
vessels  besides  capillaries,  and  are  in  most  cases  surrounded  by  a 
greater  or  less  amount  of  connective  tissue  ;  the  central  vein,  on  the 
other  hand,  is  usually  unassociated  with  other  vessels,  except  capilla- 
ries, and  surrounded  only  by  a  scarcely  appreciable  amount  of  connec- 
tive tissue. 

Injected  Liver. — The  general  arrangement  of  the  blood-vessels  is 
best  studied  in  sections  from  a  human  or  rabbit's  liver,  which  has 
been  injected  through  the  portal  vein  with  a  solution  of  gelatine 
colored  with  Prussian  blue.  The  sections  are  stained  with  carmine  or 
eosin,  and  mounted  in  Canada  balsam. 

Injected  Gall-capillaries. — By  a  careful  injection  of  the  blue  gela- 
tine mixture  into  the  hepatic  duct,  the  gall-capillaries  may  be  partially 
filled.     Sections  are  ma.de,  stained  with  eosin,  and  mounted  in  balsam. 


CHAPTER   XL 

LYMPH-NODES— SPLEEN. 

Lymphatic  Glands  or  Lymph-Nodes. 

If  we  follow  the  lymphatic  vessels  in  their  course  from  the  periphery 
toward  the  thoracic  duct,  we  find  that,  sooner  or  later,  they  are  inter- 
rupted in  their  course  by  certain  nodular  masses,  more  abundant  in 
some  parts  of  the  body  than  in  others,  and  of  variable  size,  commonly 
called  lymphatic  glands.  They  are  not  glands  in  the  limited  sense  of 
the  word,  as  it  has  been  already  defined ;  for,  so  far  as  we  know,  they 
furnish  no  specific  secretion,  they  have  no  excretory  ducts,  and  seem 
to  have  an  entirely  different  structure  and  function  from  the  glands 
proper.  It  would  be  better  to  call  them  lymph-nodes,  for  that  would 
not  be  misleading,  as  the  word  gland  is,  in  regard  to  their  relations  to 
other  structures ;  but,  as  they  are  universally  known  as  lymphatic 
glands,  we  shall  at  present  use  both  terms.  The  lymph-nodes  pre- 
sent a  great  diversity  in  form,  being  spherical,  ovoid,  discoidal,  or 
irregularly  prismatic;  they  always  present  at  one  side  a  hilum-like 
marking,  and  however  diverse  their  forms,  they  approach  more  or  less 
closely  in  shape  to  the  type  of  the  kidney.  If  we  make  a  section 
through  a  fresh  gland,  at  right  angles  to  its  long  axis  and  through  the 
hilus,  we  find  it  more  or  less  distinctly  divided  into  two  zones  :  an 
outer,  or  cortical  zone — the  cortex — which  is  soft  and  grayish  or  red- 
dish in  color,  and  divided  into  ovoidal  or  irregular-shaped  masses ; 
and  an  inner,  or  medullary  zone — the  medulla — adjacent  to  the  hilus, 
which  is  firmer,  and  has  a  more  uniform,  or  sometimes  irregularly- 
reticulated,  grayish  or  brownish  red  surface.  The  nodes  are  sur- 
rounded by  a  firm,  dense  capsule  of  connective  tissue,  with  a  few 
elastic  fibres  and  smooth  muscle-cells.  The  capsule  sends  inward 
numerous  partitions  or  trabeculse,  which  divide  the  cortex  into  a 
series  of  intercommunicating  chambers,  and  the  medulla  into  numer- 
ous irregular  connecting  passages.  These  septa  are  formed  of  the 
same  elements  as  the  capsule,  and  at  the  hilus  are  continuous  with  a 
dense  mass  of  connective  tissue,  through  which  the  blood-vessels  enter 
the  organ.  If  we  examine  the  spaces  left  between  the  septa,  we  find 
that  in  the  cortex  they  are  incompletely  filled  with  ovoidal  or  globu- 
lar bodies,  which  are  continuous  with  cord-like  anastomosing  struc- 
tures lying  in  the  narrower  and  irregular  spaces  in  the  medulla  ;  the 
bodies  in  the  cortex  are  called  lymph-follicles,  and  the  cord-like 
structures  in  the  medulla  lymph-tubes  or  lymph-cords.  If  now  we 
examine  more  minutely  the  structure  of  the  lvmph-follicles,  we  find 
6 


82  NORMAL  HISTOLOGY. 

that  they  consist,  in  the  first  place,  of  a  frame-work  of  reticular  con- 
nective tissue,  whose  meshes  are  largest  in  the  centre  of  the  follicle, 
narrower  and  smaller  in  the  periphery ;  indeed,  so  closely  crowded 
together  are  the  trabecular  here,  that  they  give  to  the  follicle  a  toler- 
ably well  defined  outline.  In  the  second  place,  the  meshes  of  the 
reticulum  are  closely  filled  with  small  spheroidal  cells,  having,  in  gen- 
eral, the  characters  of  lymph-cells  ;  in  many  of  them,  however,  the 
nucleus  is  very  large,  occupying  the  greater  part  of  the  cell.  The 
follicles — in  which  we  recognize  essentially  the  same  structure  that  we 
have  found  in  the  lymphatic  follicles  of  the  intestine — do  not  entirely 
fill  the  cavities  in  which  they  lie,  but  are  surrounded  on  all  sides  by  a 
narrow  space.  If  now  we  examine  the  relation  of  the  follicles  to  the 
walls  of  their  investing  spaces,  we  find  that,  from  the  walls  of  these 
spaces,  delicate  branching  trabecular  pass  inward  to  the  surface  of  the 
follicle,  where  they  become  continuous  with  the  reticular  tissue  of 
the  latter.  They  are,  in  fact,  themselves  reticular  connective  tissue, 
similar  to  that  of  the  follicle,  except  that  the  trabecular  are  coarser 
and  the  meshes  broader.  Stretching  across  the  space  surrounding 
the  follicles,  they  suspend  the  latter  so  that  they  hang  free  in  the 
cavities.  The  space  thus  formed  around  the  follicle  is  called  the  peri- 
follicular space,  or  better,  lymph-sinus,  for  reasons  which  will  be 
presently  given. 

If  we  now  turn  our  attention  to  the  lymph-cords  of  the  medulla, 
which,  it  will  be  remembered,   are  continuous  with  the  follicles,  we 
find  that  they  have  an  exactly  similar  structure,  and  bear  the  same  re- 
lation to  the  more  irregularly  arranged  connective-tissue  septa  which 
bound  the  branching  spaces  in  which  they  ramify,  that  the  follicles  do 
to  the  walls  of  their  investing  spaces,  i.  e.,  they  are  suspended  in  them 
by  coarse  trabecular  of  reticular  connective  tissue,  and  surrounded 
on  all  sides  by  lymph-sinuses.     These  lymph-cords,  ramifying  and  in- 
osculating in  the  medullary  portion  of  the  gland,  form  an  intricate 
system  of  intercommunications  between  all  the  follicles  of  the  gland. 
Injections    of  dilute    solutions  of  nitrate  of  silver  into   the    lymph- 
sinuses  show  that  the  surfaces  of  the  follicles  and  lymph-cords,  as  well 
as  the  walls  of  their  investing  spaces,  are  covered  with  endothelial 
cells.     If  we  study  the  relation  of  the  lymph-vessels  to  the  lymph- 
nodes,  we  find  that  the  former,  on  arriving  at  the  surface  of  the  node 
— afferent  vessels — pierce  the  capsule  and  become  continuous  with 
the  lymph-sinuses,  into  which  they  pour  their  contents ;  we  find,  fur- 
ther, that  efferent  vessels,  still  continuous  with  the  sinuses,  leave  the 
organ   at  other  points,  frequently  at  the  hilus.      The  lymph-glands, 
then,  are  structures  interrupting  the  course  of  the  lymphatic  vessels, 
in  which  the  lymph  is  forced  to  pass   through  a  series  of  irregular 
branching  spaces  or  sinuses,  which  are  crossed  by  a  net-work  of  reti- 
cular tissue,  and  bathing  in  its  course  certain  peculiar  structures — the 
follicles  and  lymph-cords — whose  function  we  do  not  yet  understand. 
The  lymph-sinuses  contain  not  only  the  fluid  of  the  lymph,  but  numer- 
ous lymph-cells,  and  usually  a  certain  number  of  red  blood-cells. 

The  principal  blood-vessels  enter  the  nodes  at  the  hilus,  and  the 
arteries,  sending  off  branches  to  the  connective  tissue  there  and  to  the 


LYMPH-NODES— SPLEEN.  83 

septa,  divide  and  subdivide,  and  soon  enter  the  lymph-cords  ;  they 
pass  along  in  the  axis  of  these,  giving  off  a  long-meshed  capillary  sys- 
tem; then  entering  the  follicles,  they  break  up  into  a  loose  capillary 
net-work,  from  which  the  blood  is  collected  into  venous  radicles,  and 
poured  into  veins  which  pass  out  through  the  lymph-cords  and  out  of 
the  organ  in  connection  with  the  arteries.  Small  blood-vessels  usually 
enter  the  capsule  at  other  points  than  the  hilus,  and  are  largely  dis- 
tributed to  the  capsule  and  'the  connective  tissue  of  the  septa.  The 
number  and  arrangement  of  the  follicles  varies  greatly  in  different 
lymphatic  glands  :  in  some  there  is  but  a  single  layer  in  the  cortex  ; 
in  others,  several  layers  are  superimposed  and  more  or  less  crowded 
— thus  giving  to  some  glands  a  narrow,  to  others  a  broad  and  volumi- 
nous cortex.  In  many  animals,  as  the  ox,  the  reticular  tissue  of  the 
medullary  portion  contains  an  abundance  of  brown  pigment. 

PRACTICAL   STUDY. 

Lymph-Sinuses  Injected. — A  general  view  of  the  lymph-sinuses 
and  their  relations  to  the  follicles  and  tubes  is  best  obtained  from 
sections  of  nodes  whose  lymph-channels  have  been  tilled  with  a  col- 
ored solution  of  gelatine.  A  gland  being  exposed  in  a  recently 
killed  animal — one  of  the  cervical  glands  of  the  dog  answers  well — 
a  hypodermic  syringe  with  a  sharp  canula  is  warmed  and  filled  with 
a  warm  solution  of  gelatine  colored  with  Prussian  blue  ;  the  canula  is 
thrust  through  the  capsule  at  any  point  and  the  fluid  injected.  The 
node  will  become  mottled  with  blue,  and  the  mass  will  often  be  seen 
to  flow  into  adjacent  glands.  If  it  be  not  desired  to  inject  more  than 
one,  a  ligature  should  be  passed  around  the  vessels  leading  to  the 
others.  When  a  sufficient  quantity  of  the  fluid  has  been  injected  to 
render  the  gland  firm  and  the  capsule  tense,  the  canula  is  withdrawn, 
and  the  node  cooled  by  a  piece  of  ice  laid  upon  it,  or  by  a  stream  of 
cold  water.  When  the  gelatine  has  solidified,  the  gland  is  cut  out, 
divided  longitudinally,  and  put  into  strong  alcohol.  When  it  has  be- 
come sufficiently  hard,  sections  are  made  through  the  entire  gland, 
stained  with  picrocarmine  or  eosin,  and  mounted  in  balsam. 

Pencilled  Section  of  a  Lymph-node. — The  meshes  of  the  follicles 
and  cords,  as  well  as  the  lymph-sinuses,  are  so  closely  crowded  with 
cells,  that  the  structure  of  the  supporting  tissue  cannot  be  clearly 
made  out,  even  in  very  thin  sections,  without  removing  some  of  the 
cells.  This  may  be  done  by  shaking  in  water,  as  in  the  preparation 
of  reticular  connective  tissue  (p.  34),  or,  better,  by  pencilling  under 
water  with  a  soft  camel' s-hair  pencil  in  the  way  suggested  by  Ranvier. 
An  interstitial  injection  of  a  fresh  gland  with  one  per  cent,  solution  of 
osmic  acid  is  made,  in  the  way  described  on  page  ^^,  and  the  gland 
laid  for  an  hour  in  water,  and  then  immersed  for  three  days  in  strong 
alcohol.  As  it  is  not  usually  sufficiently  hardened  by  the  alcohol  to 
admit  of  very  thin  sections  being  made,  it  is  now  to  be  laid  for  a  few 
moments  in  water,  and  then  in  a  syrupy  solution  of  gum-arabic,  in 
which  it  remains  for  twenty-four  hours,  and  then  transferred  to  alco- 
hol.    This  precipitates   the  gum   in  the   interstices,   and   gives  it  a 


84  NORMAL   HISTOLOGY. 

proper  consistence  for  cutting.  Thin  sections,  made  so  as  to  em- 
brace both  cortex  and  medulla,  are  transferred  to  a  shallow,  flat-bot- 
tomed dish  of  water,  and  as  the  gum  begins  to  swell,  the  section 
should  be  laid  flat  and  gently  pressed  down  with  a  camel' s-hair  pen- 
cil, whose  end  is  square,  so  that  it  shall  adhere  at  all  points  to  the 
bottom  of  the  dish.  When  this  is  done,  the  specimen  should  be 
gently  tapped  with  the  brush  held  perpendicularly ;  as  the  cells  are 
freed  in  this  way,  they  float  off  into  the  Water,  and  the  specimen  will 
be  seen  to  become  more  and  more  transparent.  When  the  operation 
is  completed — which  will  be  determined  by  the  extreme  delicacy 
which  the  specimen  will  finally  present  when  most  of  the  cells  are 
removed — it  is  floated,  without  folds,  on  to  a  slide,  stained  with  hema- 
toxylin, and  mounted  in  glycerine. 

Blood-vessels. — To  obtain  an  injection  of  the  blood-vessels  of  the 
lymph-nodes,  either  a  whole  animal,  such  as  a  rabbit  or  dog,  may  be 
injected,  through  the  aorta,  with  the  blue  gelatine  mixture,  or  a  single 
gland,  such  as  the  cervical  or  mesenteric,  through  its  main  artery. 

The  glands  should  be  hardened  in  alcohol,  and  the  sections  stained 
with  eosin  and  mounted  in  Canada  balsam. 


The  Spleen. 

The  spleen,  although  differing  in  many  important  particulars  from 
the  lymphatic  glands,  vet  presents  many  striking  analogies  with  them. 
Like  them,  it  presents  on  cross-section,  to  the  naked  eye,  a  fibrous 
envelope — the  capsule ', — from  which  septa  and  trabecular  pass  into  the 
organ  enclosing  irregular  spaces.  Here  also  we  find  the  spaces  be- 
tween the  septa  filled  with  a  soft  substance  presenting  two  distinct 
modes  of  arrangement ;  we  find  first,  irregularly  scattered  through  the 
organ,  small  grayish  globular  structures  called  Malpighian  bodies  or 
follicles ;  and  second,  between  these,  filling  up  the  remaining  space 
between  the  trabecular,  a  soft  red  tissue  called  the  pulp. 

Finally,  we  find  blood-vessels  entering  the  organ  at  the  hilus.  We 
have  then,  in  studying  the  spleen,  to  consider  the  connective-tissue 
capsule  and  trabecules,  the  follicles,  the  pulp,  and  the  blood-vessels. 

The  capside  of  the  spleen  consists  of  a  dense  envelope  of  inter- 
lacing connective-tissue  fibres  with  flattened  cells,  with  a  large  number 
of  fine  elastic  fibres  and  a  few  smooth  muscle-cells.  It  is  covered  by 
a  layer  of  endothelial  cells  similar  to  those  of  the  general  peritoneal 
surface.  At  the  hilus  it  passes  inward,  forming  a  sheath  for  the  large 
vessels,  and  joins  a  complicated  system  of  septa  and  trabecules,  which, 
proceeding  inward  from  all  parts  of  the  capsule,  form  a  multitude  of 
irregular  communicating  spaces  in  which  the  follicles  and  splenic  pulp 
lie.  These  trabecular  and  septa  are  made  up  of  the  same  elements  as 
the  capsule,  and  in  size  and  abundance  vary  greatly  in  different  ani- 
mals, being  in  man  only  moderately  developed.  The  follicles,  or 
Malpighia7i  bodies,  have  essentially  the  same  structure  as  the  follicles 
of  the  lymph-nodes — that  is,  they  are  formed  of  a  small  mass  of  sup- 
porting reticular  connective-tissue,  whose  meshes  are  narrowest  at  the 
periphery,  and  closely  filled  throughout  with  small  spheroidal  cells, 


LYMPH-NODES — SPLEEN.  85 

and  supplied  with  a  net-work  of  capillaries  ;  we  usually  find  here,  how- 
ever, a  small  artery  passing  either  through  the  centre  or  at  one  side  of 
the  follicle.  In  order  to  fully  understand  the  follicles  of  the  spleen, 
it  is  necessary  to  study  their  relation  to  the  arteries,  in  which  respect 
they  seem  entirely  to  differ  from  their  analogues  in  the  lymph-nodes. 
The  arteries  and  veins  as  they  enter  the  hilus  of  the  spleen  are  sur- 
rounded, as  above  mentioned,  by  a  connective-tissue  sheath  ;  after 
passing  for  a  short  distance  inward,  they  separate,  and  the  arteries, 
still  accompanied  by  a  certain  amount  of  connective  tissue,  rapidly 
divide  and  subdivide,  proceeding  farther  inward,  until  the  small  branches 
finally  break  up  into  brush-like  bundles  of  delicate  twigs.  If  now  we 
carefully  study  the  walls  of  the  smaller  arteries,  we  find  that  in  certain 
parts  they  undergo  a  singular  modification :  in  the  larger  trunks  the 
connective-tissue  sheath,  in  the  small  twigs  the  adventitia,  become 
very  loose  in  texture  and  their  meshes  become  filled  with  spheroidal 
cells  resembling  lymph-cells — this  is  called  lymphoid  infiltration  of  the 
arteries  ;  then  we  find  that  at  certain  points  of  the  small  twigs  this  in- 
filtration suddenly  becomes  quite  extensive,  the  intercellular  substance 
assuming  the  character  of  reticular  connective  tissue  ;  and  distinct  cir- 
cumscribed swellings  are  formed  either  around  or  at  one  side  of  the 
arteries — these  are  the  splenic  follicles  or  Malpighian  bodies.  In 
some  animals  this  infiltration  is  quite  extensive  and  continues  along 
the  arteries  for  considerable  distances  at  either  side  of  the  follicles  ■  in 
others,  as  in  man,  it  is  not  very  marked  except  in  the  follicles,  but 
may  frequently  be  seen  along  the  arteries  adjacent  to  them,  in  the  form 
of  narrow  cellular  sheaths.  The  capillary  net-work  of  the  follicles  is 
connected  with  arterial  twigs  which  either  penetrate  from  without, 
or  are  given  off  from  the  follicular  artery  as  it  passes  through  the 
body. 

Let  us  now  turn  to  the  pulp.  This  is  composed,  in  the  first  place, 
of  a  multitude  of  irregular,  frequently  anastomosing  cords — called pulp- 
cords — between  which,  and  bounded  closely  by  them,  lie  in  the  second 
place,  a  series  of  branching  channels — the  cavernous  veins.  The  pulp- 
cords — joined  on  the  one  hand  to  the  follicles  and  infiltrated  arterial 
sheaths,  and  on  the  other  to  the  connective-tissue  septa — consist  of  a 
frame-work  of  delicate  reticular  connective  tissue,  continuous  with  the 
sustaining  tissue  of  the  follicles,  the  meshes  of  which  are  incompletely 
filled  with  various  kinds  of  cells.  Among  these  cells  we  find  sphe- 
roidal cells,  like  lymph-cells  ;  larger  colorless  cells  with  one  or  more 
large  nuclei ;  red  blood-cells  ;  fragments  of  red  blood-cells ;  larger 
and  smaller  colorless  cells  containing  pigment  in  various  forms;  and 
finally,  there  are  sometimes  found  in  varying  numbers  cells  which  re- 
semble the  lymph  and  larger  colorless  cells  in  form,  but  whose  bodies, 
either  homogeneous  or  granular,  have  a  color  similar  to  that  of  the 
red  blood-cells.  These  latter  cells,  like  certain  similar  cells  already 
mentioned  as  occurring  in  the  marrow  of  bones,  are  regarded  by  many 
observers  as  intermediate  forms  between  the  colorless  and  the  red 
blood -cells,  and  are  often  called  nucleated  red  blood-cells.  Although 
many  recent  observations  would  tend  to  confirm  this  idea,  their  nature 
is  as  yet  by  no  means  absolutely  determined.     We  have  still  to  con- 


S6  NORMAL   HISTOLOGY. 

sider  the  structure  of  the  second  constituent  of  pulp — the  cavernous 
veins.  Following  the  splenic  veins  inward  from  the  hilus,  we  find 
that  they  gradually  lose  their  connective-tissue  sheath,  and  then  their 
outer  coats,  and  then  rapidly  divide  and  subdivide  to  form  a  multitude 
of  intercommunicating  thin-walled  canals  of  tolerably  uniform  calibre, 
which  occupy  the  irregular  branching  spaces  between  the  pulp-cords. 
These  ultimate  venous  trunks  are  called  cavernous  veins,  and  their 
walls  consist  of  little  else  than  a  few  widely  separated  branching  circu- 
lar and  oblique  fibres,  upon  which  lie,  at  varying  intervals,  elongated, 
curved,  spindle-shaped  cells,  which  have  their  long  axis  parallel  with 
the  axis  of  the  canal.  These  veins,  whose  walls,  as  will  be  seen,  are 
not  closed  but  fenestrated,  are  in  direct  communication  with  the  spaces 
which  are  still  left  in  the  meshes  of  the  pulp-cords  by  the  cells  which 
incompletely  fill  them. 

Still  another  point  remains  to  be  considered,  namely,  the  course 
of  the  blood  after  its  exit  from  the  above-described  fine  arterial  twigs 
on  which  the  Malpighian  bodies  are  formed,  until  it  enters  the  fenes- 
trated cavernous  veins  of  the  pulp.  The  opinion  of  different  observ- 
ers on  this  point  differs  somewhat,  owing  to  the  extreme  technical 
difficulties  in  the  investigation;  but  it  is  believed  by  the  majority  of 
histologists  at  present,  that  after  passing  out  of  the  fine  arterial  twigs, 
through  the  intervention  of  capillaries,  it  is  poured  directly  into  the 
meshes  of  the  pulp-cords,  and  that  after  circulating  here  around  the 
cells  without  distinctly  walled  channels,  it  finally  finds  its  way  through 
their  fenestrated  walls  into  the  cavernous  veins,  whence  it  passes  out 
of  the  organ  through  the  large  efferent  veins. 


PRACTICAL    STUDY. 

Sections  of  U?iinjected  Spleen. — A  general  view  of  the  arrangement 
of  the  different  structures  found  in  the  spleen  may  be  obtained  from 
very  thin  sections  of  a  spleen  which  has  been  placed  for  ten  days  in 
two  per  cent,  solution  of  bichromate  of  potash,  and  then  transferred 
to  alcohol  and  allowed  to  remain  until  hard  enough  to  cut.  The 
sections  are  stained  with  hematoxylin  and  eosin,  and  mounted  in 
glycerine. 

Sections  of  Lijecied  Spleen. — A  spleen  is  injected  through  the 
splenic  artery  with  the  blue  gelatine  mixture,  hardened  in  alcohol, 
and  thin  sections  made  and  stained  with  carmine.  In  such  speci- 
mens, if  successful,  the  blue  fluid  will  be  found  not  only  in  the  blood- 
vessels, but  also  in  patches  in  the  pulp-tissue,  where  it  will  be  seen 
lying  between  the  cells,  looking  like  an  accidental  extravasation  :  its 
presence  here  is  explained,  however,  by  the  structure  of  the  organ. 

Pencilled  Sections  of  Spleen. — A  bit  of  human  spleen  is  put  for  a 
week  in  two  per  cent,  solution  of  bichromate  of  potash,  washed  and 
transferred  first  to  dilute,  then  to  strong  alcohol;  when  sufficiently 
hard,  very  thin  sections  are  made,  carefully  pencilled  under  water  (see 
page  83).  stained  with  haematoxylin,  and  mounted  in  glycerine. 

Isolated  Cells. — A  bit  of  the  fresh  organ  is  teased  on  a  slide  in  half 
per  cent,  salt  solution,  and  studied  in  the  same.     Should  the  red  blood- 


LYMPH-NODES— SPLEEN.  87 

cells,  as  is  apt  to  be  the  case,  be  so  numerous  as  to  conceal  the  other 
forms,  a  drop  or  two  of  dilute  alcohol  (1  to  2)  is  allowed  to  run  un- 
der the  cover-glass,  and  after  a  few  moments  the  red  blood-cells  will 
have  become  so  transparent  as  not  seriously  to  interfere  with  the  ob- 
servation. If  the  nuclei  are  not  sufficiently  well  denned,  a  drop  of 
eosin  may  be  added  after  the  alcohol.  In  addition  to  the  above  de- 
scribed cells  of  the  pulp-cords  in  teased  specimens,  narrow,  elongated, 
often  curved  cells,  with  projecting  nuclei,  are  frequently  seen  ;  these 
are  the  above-described  lining  cells  of  the  cavernous  veins. 


CHAPTER   XII. 

SUPRA-RENAL  CAPSULES— THYROID  GLAND. 

Supra-renal  Capsules. 

In  sections  of  the  supra-renal  capsules,  through  their  thickest  part, 
and  at  right  angles  to  the  long  axis  of  the  organ,  we  see  with  the  naked 
eye  two  distinct  layers :  a  tolerably  firm,  yellowish,  striated  cortical 
layer  of  considerable  thickness — the  co7'tex,  and  a  narrower  central,  yel- 
low or  reddish  medullary  portion — the  medulla.  Between  the  two,  or 
rather  at  the  inner  side  of  the  cortical  portion,  a  more  or  less  distinct 
brown  zone  is  seen.  If  we  examine  thin  sections  microscopically,  we 
find  that  the  organ  is  enclosed  in  a  firm  connective-tissue  capsule,  in 
which  are  numerous  elastic  fibres.  From  this  capsule  delicate,  con- 
verging connective-tissue  bands,  or  septa,  pass  inward,  and  being 
joined  together  by  delicate  transverse  bands  or  trabecular,  divide  the 
cortex  into  a  multitude  of  variously  shaped  chambers.  In  the  peri- 
phery, just  beneath  the  capsule,  the  chambers  are  small  and  irregular 
in  shape  ;  then  comes  a  broader  zone,  whose  chambers  are  long  and 
narrow ;  and  again,  at  the  inner  border  of  the  cortex,  they  are  small 
and  irregular.  In  the  medulla  the  supporting  frame-work  has  the  form 
of  a  delicate  reticulum  with  small  irregular  meshes.  The  spaces  thus 
formed  are  filled  with  cells,  which  differ  in  character  in  the  different 
parts  ;  in  the  cortex  these  parenchyma  cells  are,  for  the  most  part, 
large,  polyhedral,  and  granular ;  less  abundant  are  smaller  cuboidal  or 
cylindrical  forms.  Those  in  the  elongated  chambers  are  not  infre- 
quently crowded  with  fat-droplets,  and  those  lying  in  the  inner  zone 
usually  contain  an  abundance  of  brown  pigment,  forming  the  above- 
mentioned  brown  zone.  The  meshes  of  the  medulla  are  filled  with 
large,  globular,  or  angular  or  branched,  finely  granular  cells,  which,  in 
marked  contrast  to  the  cortical  cells,  are  stained  intensely  brown  by 
solutions  of  chromic  acid  or  its  salts.  The  blood-vessels  are  very 
abundant ;  many  of  them  have  extremely  thin  walls  and  lie  in  close 
contact  with  the  cells  of  the  parenchyma.  The  organ  is  abundantly 
supplied  with  nerves,  which,  passing  along  the  converging  trabecular, 
form  a  dense  plexus  in  the  medulla,  in  which,  as  well  as  in  the  cap- 
sule, considerable  numbers  cf  ganglion-cells  are  found. 

PRACTICAL    STUDY. 

Sections. — A  human  supra-renal  capsule,  as  fresh  as  possible — or, 
if  this  cannot  be  obtained,  that  of  the  guinea-pig  or  ox — is  cut  trans- 


SUPRA-RENAL   CAPSULES— THYROID    GLAND.  89 

versely  into  two  or  three  pieces  and  placed  in  two  per  cent,  solution 
of  bichromate  of  potassium  for  ten  days,  and  the  hardening  is  then 
completed  in  alcohol.  Transverse  sections  are  made,  stained  with 
hsematoxylin  and  eosin,  and  mounted  in  glycerine  or  balsam. 

Another  section  should  be  shaken  in  water  in  a  test-tube  to  free 
the  parenchyma-cells,  stained  with  hematoxylin,  and  mounted  in 
glycerine. 

The  Thyroid  Gland. 

The  thyroid  gland  is  composed  of  a  congeries  of  small  spheroidal  or 
irregular-shaped  vesicles  or  alveoli,  enclosed  in  connective  tissue,  and 
grouped  together  to  form  lobules.  Each  alveolus  is  surrounded  by  a 
delicate  membrana  propria,  and  lined  with  a  single  layer  of  cuboidal 
cells.  The  alveoli  rue  entirely  separate  from  one  another,  and  the 
gland  has  no  excretory  duct.  The  connective  tissue  surrounding 
the  alveoli  contains  numerous  blood-vessels.  The  entire  gland,  which 
consists  of  two  lateral  lobes,  united  at  their  lower  extremities  by  a 
transverse  commissure,  is  enclosed  in  a  dense  connective-tissue  en- 
velope. The  alveoli  are  filled  with  a  clear,  homogeneous  fluid,  which 
in  adult  life  is  frequently  transformed  into  or  replaced  by  a  translu- 
cent material,  commonly  called  colloid.  Owing  to  the  pressure  which 
the  accumulating  colloid  substance  exerts  on  the  epithelium  lining  the 
alveoli,  they  are  often  very  much  flattened,  and  not  infrequently  al- 
most entirely  disappear.  The  colloid  material  seems  to  be  formed, 
in  part  at  least,  by  a  transformation  of  the  contents  of  the  epithelial 
cells,  and  its  formation  under  pathological  conditions  gives  rise  to 
one  of  the  forms  of  goitre.  The  fact  that  the  thyroid  has  no  excre- 
tory ducts,  although  otherwise  having  the  structure  of  a  gland,  gives 
it  a  unique  position  among  those  organs. 


PRACTICAL    STUDY. 

The  thyroid  of  a  child  or  adult — better  the  former — is  cut  into 
small  pieces  and  put  for  a  day  into  dilute,  and  then  into  strong  alco- 
hol. When  sufficiently  hard,  thin  sections  are  made,  stained  with 
hematoxylin  and  eosin,  and  mounted  in  balsam. 


CHAPTER   XIII. 

THE   RESPIRATORY   APPARATUS. 

The  respiratory  apparatus  consists  of  a  multitude  of  small  cavities 
or  chambers,  on  whose  walls  the  blood  is  brought  into  close  contact 
with  the  air,  and  of  a  system  of  branching  tubes  through  which  the 
air  is  conducted  to  and  from  them.  The  conducting  tubes  are  the 
larynx,  the  trachea,  and  the  bronchi.  We  shall  limit  our  study  of  the 
tubes  to  the  two  last. 

The  walls  of  the  trachea  consist  of  several  layers ;  commencing  at  the 
outside, — we  have,  first,  a  layer  of  firm  connective  tissue  —  the  fibrous 
laye?' — in  which  lie  embedded,  incomplete  cartilaginous  rings,  the  space 
between  whose  free  ends,  at  the  posterior  portion  of  the  tube,  is  bridged 
over  by  transverse  bands  of  smooth  muscular  tissue,  which  bind  the 
ends  together.  The  cartilaginous  rings  are  of  the  hyaline  variety,  and 
their  perichondrium  is  continuous  with  the  looser  connective  tissue  of 
the  layer  in  which  they  lie.  This  layer  merges  into  the  next,  the  sub- 
??iucosa,  which  consists  of  fibrillar  connective  tissue  with  elastic  fibres, 
the  latter  running  chiefly  in  a  longitudinal  direction. 

Still  farther  inward,  and  not  distinctly  separated  from  the  submu- 
cosa,  lies  the  mucosa,  composed  of  fibrillar  connective  tissue  contain- 
ing an  abundance  of  variously  shaped  cells,  and,  for  the  most  part, 
longitudinally  arranged  elastic  fibres.  The  connective  tissue  of  the 
mucosa,  in  some  parts  of  the  trachea,  does  not  form  a  uniform  layer, 
but  is  arranged  in  more  or  less  well-defined  longitudinal  bundles,  giv- 
ing the  surface  a  wavy  or  folded  appearance.  Internally  the  mucosa 
is  bordered  by  a  thin,  homogeneous  membrane,  the  basal  membraiie, 
upon  which  the  epithelial  cells  lining  the  trachea  rest.  The  epithelial 
cells  are  usually  arranged  in  about  three  layers.  Lying  upon  the  basal 
membrane  are  irregularly  spheroidal,  often  somewhat  elongated  cells  ; 
upon  these  lie  fusiform  or  pear-shaped  cells,  while  the  surface  is  formed 
by  a  layer  of  pyramidal  or  cylindrical  ciliated  cells.  In  the  submucosa, 
and  sometimes  extending  outward  along  and  between  the  cartilaginous 
rings,  lie  racemose  mucous  glands,  whose  excretory  ducts,  lined  with 
cylindrical  epithelium,  pass  obliquely  inward  and  terminate  on  the 
surface  in  expanded  orifices.  The  alveoli  of  the  glands  are  lined  with 
a  single  layer  of  slightly  granular,  spheroidal  cells.  When  the  glands 
are  in  a  condition  of  functional  activity,  however,  the  cells  become 
larger,  their  outlines  indistinct,  their  nuclei  are  crowded  to  one  side,  and 
their  contents  are  apparently  transformed  into  a  homogeneous  mucous 
mass.  Scattered  here  and  there  among  the  glands,  and  between  the 
cartilaginous  rings,  lie  larger  and  smaller  clusters  of  fat-cells.     The 


THE   RESPIRATORY  APPARATUS.  9 1 

blood-vessels,  passing  through  the  outer  layers,  furnish  an  abundant 
•capillary  net-work  to  the  mucous  glands,  and  spread  out  in  a  rich 
capillary  plexus  beneath  the  basal  membrane. 

Although  the  larger  and  medium-sized  bronchi  have  the  same  gen- 
eral structure  as  the  trachea,  still  we  find,  aside  from  a  decrease  in 
the  thickness  of  the  walls,  certain  noteworthy  structural  modifications 
which  chiefly  concern  the  cartilaginous  and  muscular  elements.  The 
cartilage  occurs  in  the  form  of  regular,  incomplete  rings,  only  in  the 
upper  portion  of  the  larger  bronchi.  As  the  tubes  become  smaller 
the  cartilage  occurs  in  the  form  of  scattered,  irregular-shaped,  often 
angular  plates,  which  become  gradually  smaller  and  thinner.  Farther, 
a  distinct  layer  of  smooth  muscular  tissue,  in  the  form  of  transverse 
rings,  connected  with  each  other  by  interlacing  cells,  appears  between 
the  mucosa  and  submucosa.  We  find,  also,  that  the  longitudinal 
bundles  of  fibrillar  and  elastic  connective  tissue  in  the  mucosa  are 
more  strongly  developed  as  the  bronchi  become  smaller,  so  as  to  throw 
the  mucous  membrane  into  pronounced  longitudinal  folds.  Following 
the  bronchi  now  down  toward  their  finer  ramifications,  we  find  that  the 
outer  connective-tissue  layer  becomes  thinner,  the  cartilaginous  plates 
become  smaller  and  more  infrequent,  and  finally  altogether  disappear; 
the  mucous  glands  too,  after  becoming  smaller  and  simpler  in  structure, 
disappear  with  the  cartilages.  The  muscular  rings  assume,  gradually, 
the  form  of  an  uniform  thin  layer  of  transversely  arranged  muscle-cells, 
intermingled  with  elastic  fibres,  and  are  finally  represented  only  by  a 
few  scattered  transverse  cells.  The  mucosa  in  the  smaller  tubes  be- 
comes gradually  thinner,  and  finally  merges  into  the  fibrous  layer  with 
the  intervention  only  of  a  few  scattered  muscle-cells  ;  the  lower  layers 
of  epithelium  gradually  disappear,  leaving  a  single  row  of  ciliated  cells 
upon  the  basal  membrane.  Finally,  we  have,  in  the  smallest  tubes, 
very  thin  connective-tissue  walls,  containing  a  few  muscle-cells  and 
elastic  fibres,  and  lined  with  cuboidal,  ciliated,  and  finally  with  flattened 
non-ciliated  cells.  We  are  thus  led  to  the  respiratory  cavities  of  the 
lungs — the  air-chambers.  If  we  look  at  the  surface  of  a  lung  we  see 
that  it  is  divided,  by  narrow  branching  lines,  into  irregular  polygonal 
spaces,  each  one  of  which  corresponds  to  a  pulmonary  lobule.  These 
lobules  have,  on  the  surface  of  the  lung,  where  they  are  more  uniform 
in  shape  than  within,  a  pyramidal  form,  and  are  separated  by  narrow 
connective-tissue  septa,  and  each  lobule  is  in  fact  a  group  of  air-vesi- 
cles, which  are  in  communication  with  a  single  small  bronchus  and 
its  terminal  twigs.  As  the  small  bronchus  enters  the  lobule  it  divides 
into  several  terminal  twigs  ;  each  one  of  these  opens  at  its  peripheral 
extremity  into  two  or  three  irregular  tubular  passages,  called  alveolar 
passages;  on  all  sides  of  these  alveolar  passages  still  other  irregular, 
sometimes  branched  elongated  cavities,  called  infundibula,  open  out, 
and  their  walls,  as  well  as  the  walls  of  the  alveolar  passages,  are  beset 
with  tiny,  irregular-shaped  vesicles,  called  air-vesicles  or  alveoli,  which 
communicate  with  the  cavities  of  the  infundibula  and  alveolar  passages 
by  wide,  irregular  openings.  It  is  in  the  walls  of  these  air-vesicles  that 
that  interchange  of  material  between  air  and  blood  occurs  which  is  the 
essential  factor  in  respiration. 


92  NORMAL   HISTOLOGY. 

We  have  now  to  consider  the  structure  of  the  walls  of  the  alveolar 
passages,  infundibula,  and  air-vesicles.  The  gradual  thinning  which 
we  have  observed  in  the  walls  of  the  bronchi  as  they  approach  their 
terminations,  is  still  more  marked  as  we  pass  over  into  the  alveolar 
passages  and  infundibula.  Here  the  walls  consist  of  little  else  than  a 
thin,  delicately  striated,  membranous,  basement  substance,  in  which 
numerous  elastic  fibres  ramify  and  a  few  smooth  muscle-cells  are  em- 
bedded, the  whole  being  lined  with  flattened  cells.  At  the  openings 
of  the  infundibula  and  air-vesicles  the  elastic  fibres  are  grouped  to 
form  a  projecting  ring,  which  bounds  the  opening.  From  these  rings 
of  elastic  fibres  which  surround  the  openings  into  the  alveoli,  other 
elastic  fibres  are  given  off,  which,  dividing  dichotomously,  stretch  over 
the  walls  of  the  air-vesicles  in  the  form  of  a  wide-meshed  net.  the 
spaces  between  the  fibres  being  occupied  by  an  extremely  thin,  struc- 
tureless membrane,  in  which  lies  an  occasional  oval  nucleus.  The  al- 
veoli, in  the  adult,  are  lined  with  a  single  layer  of  flattened,  polygonal, 
epithelial  cells.  These  are  of  two  kinds  :  first,  small,  granular,  nucle- 
ated cells  ;  and  second,  cells  which  are  larger,  more  irregular  in 
form,  very  thin  and  transparent,  and  usually  without  nuclei.  The 
relative  proportion  of  these  two  kinds  of  cells  is  variable,  and  their 
outlines,  especially  those  of  the  larger  cells,  it  is  very  difficult  to  see 
without  resorting  to  silver-staining. 

In  the  fcetus  the  air-vesicles  are  lined  at  first  with  a  distinct  layer 
of  cylindrical  or  cuboidal  epithelial  cells,  which  become  gradually 
flattened,  and  when  respiration  is  established  at  birth  assume  the  form 
of  a  layer  of  very  thin  polygonal  cells  ;  these  change  their  character 
as  the  animal  matures,  until  in  adult  life  we  have  the  forms  above 
described. 

Just  beneath  the  epithelial  cells,  and  separated  by  them  alone 
from  the  air  within  the  vesicles,  lies  the  rich  blood-capillary  net-work, 
which,  in  bulk  as  in  importance,  is  the  most  essential  element  in  the 
walls  of  the  vesicles.  The  lungs  are  supplied  with  blood  through  the 
pulmonary  and  the  bronchial  arteries.  The  blood  from  the  latter  is 
chiefly  distributed  to  the  walls  of  the  bronchi  and  larger  blood-vessels, 
and  to  the  connective  tissue  of  the  lungs.  A  large  part  of  the  blood 
from  the  bronchial  arteries,  after  passing  through  various  sets  of 
capillaries,  returns  through  the  bronchial  veins  ;  but  a  certain  portion 
of  it  finds  its  way  into  the  pulmonary  veins  ;  indeed,  the  two  sets  of 
vessels  seem  to  be  in  communication  in  various  parts  of  the  lungs. 
The  pulmonary  artery,  following  the  course  of  the  bronchi,  divides 
and  subdivides,  until  on  reaching  the  lobules  the  small  trunks  break 
up  into  smaller  branchlets,  which  pass  along  the  alveolar  passages 
and  in  the  interlobular  connective  tissue  to  the  air-vesicles,  where  they 
break  up  into  the  rich  capillary  net- work  which  is  spread  over  their 
walls.  The  capillaries  wind  over  the  free  edges  of  the  alveolar  walls, 
and  often  project  somewhat  into  their  cavities.  The  net-work  on  the 
alveolar  walls  is  quite  dense,  and  the  vessels  are  very  broad,  leaving 
only  small  oval  or  rounded  spaces  between  them.  A  single  net-work 
frequently  supplies  the  walls  of  adjacent  vesicles,  which  in  such  cases' 
are  merged  into  one.     The  blood  passes  from  the  vesicles  into  the 


THE   RESPIRATORY   APPARATUS.  93 

pulmonary  veins  in  the  interlobular  connective  tissue,  and  then  into 
larger  trunks  which,  passing  inward,  follow  the  course  of  the  other 
large  vessels.  The  epithelial  cells  of  the  alveoli  often  contain  brown 
or  black  pigment,  and  pigment  deposits  are  of  the  most  frequent  oc- 
currence in  the  adult  lung,  in  the  interlobular  connective  tissue,  and 
in  the  connective  tissue  and  the  lymph-nodes  at  the  base  of  the  lungs. 


PRACTICAL    STUDY. 

Trachea. — A  portion  of  the  trachea  is  placed  for  ten  days  in  a  one- 
sixth  per  cent,  solution  of  chromic  acid,  washed,  and  the  hardening 
completed  in  alcohol.  A  small  bit  is  cut  out  and  embedded  in  wax 
or  hardened  liver — care  being  taken  not  to  scrape  off  the  epithelial 
cells  in  the  manipulations — longitudinal  and  transverse  sections  made, 
stained  with  haematoxylin  and  eosin,  and  mounted  in  balsam. 

Uninjccted  Lung. — The  lungs  from  the  human  subject,  or  any 
small  animal,  such  as  the  dog,  rabbit,  or  cat,  having  been  carefully 
removed,  a  canula  is  tied  into  the  trachea  or  one  of  the  large  bronchi, 
if  one  lung  only  is  to  be  prepared,  and  the  organ  is  distended  by 
pouring  a  one-sixth  per  cent,  solution  of  chromic  acid  into  the  canula, 
which,  for  this  purpose,  may  be  connected  by  a  rubber  tube  with  a 
funnel.  When  the  lung  is  filled  and  its  surface  tense,  the  trachea  or 
bronchus  is  tied,  and  the  entire  organ  immersed  in  the  same  fluid  with 
which  its  alveoli  are  filled.  After  a  couple  of  days  the  organ  may  be 
cut  in  pieces  and  put  into  a  stronger  solution  of  the  chromic  acid, 
one-fifth  to  one-fourth  per  cent.  In  three  or  four  days  they  are 
washed  and  transferred  at  first  to  dilute,  and  then  to  strong  alcohol. 
A  small  piece,  not  more  than  a  cubic  centimetre  in  size,  in  which  one 
or  more  small  bronchi  are  seen,  is  cut  out  and  embedded  in  cacoa 
butter  in  the  following  way  :  the  bit  of  tissue  is  transferred  from  the 
alcohol  to  a  small  dish  of  oil  of  cloves,  which  in  a  short  time  will 
penetrate  the  tissue  and  render  it  transparent.  It  is  then  placed  in 
melted  cacoa  butter  whose  temperature  is  kept  just  above  its  melting- 
point  on  a  water-bath,  and  allowed  to  remain  for  a  couple  of  hours. 
The  bit  of  lung  is  then  put  into  a  little  paper  box,  made  for  the  pur- 
pose from  writing-paper,  in  such  a  position  that  transverse  sections  of 
some  of  the  small  bronchi  can  be  conveniently  made,  and  the  melted 
cacao  butter  poured  in  around  it.  It  is  set  in  a  cool  place,  and,  when 
hard,  sections  are  made  in  the  usual  way  in  alcohol.  By  this  pro- 
cedure the  air-vesicles  and  all  the  interstices  of  the  tissue  are  filled 
with  a  solid  material  of  excellent  consistence  for  cutting,  and  the 
thinnest  of  sections  can  be  made  from  the  most  delicate  of  tissues 
when  thus  prepared. 

In  order  to  remove  the  cacao  butter  the  sections  are  placed  in  oil 
of  cloves  and  gently  warmed,  when  the  fat  will  readily  dissolve  and 
the  sections  again  become  transparent.  They  are  then  transferred  to  al- 
cohol, stained  with  haematoxylin  and  eosin,  and  mounted  in  glycerine. 
They  may  be  permanently  preserved  in  glycerine  or  mounted  in 
Canada  balsam. 

Epithelium  of  the  Air-vesicles. — Ten   grammes    of  gelatine  are 


94  NORMAL   HISTOLOGY. 

added  to  80  c.c.  of  pure  water,  and  after  soaking  for  half  an  hour, 
melted  on  a  water-bath  ;  two  decigrammes  of  nitrate  of  silver  are 
dissolved  in  a  few  drops  of  warm  water  and  mixed  with  the  warm 
gelatine.  A  canula  is  now  tied  into  the  trachea  of  the  lungs  of  a 
young  animal  and  attached  by  a  rubber  tube  to  a  warm  glass  funnel. 
The  warm  gelatine-silver  mixture  is  now  poured  into  the  funnel  and 
allowed  to  run  into  the  lungs  until  they  are  well  distended,  the  pres- 
sure being  regulated  by  the  height  at  which  the  funnel  is  held  above 
the  lungs.  A  ligature  is  now  passed  around  the  trachea  below  the 
canula,  and  the  latter  removed.  The  lungs  are  set  aside  in  a  cool 
place  until  the  gelatine  solidifies,  when  thin  sections  are  made  with  a 
razor  moistened  with  wrater,  mounted  at  once  in  glycerine,  and  ex- 
posed to  the  light  until  they  become  brown.  They  are  studied  with 
high  powers  unstained,  or  are  stained  with  dilute  eosin. 

Injected  Lung. — From  bits  of  lung  whose  blood-vessels  have  been 
injected  with  the  blue  gelatine  mixture  through  the  pulmonary  artery, 
sections,  which  need  not  be  very  thin,  are  made,  stained  with  carmine 
or  eosin,  and  mounted  in  balsam. 


CHAPTER  XIV. 

THE  KIDNEY. 

If  a  longitudinal  section  be  made  through  the  middle  of  a  rabbit's 
kidney  from  the  convex  border  toward  the  hilus,  the  cut  surface  will 
present  a  well-marked  separation  into  an  outer  cortical  and  inner 
medullary  substance ;  between  these  two  are  seen  sections  of  large 
blood-vessels.  The  medullary  portion,  called  medullary  pyramid  or 
medulla,  and  occupying  the  central  part  of  the  organ,  terminates  in 
the  pelvis  in  a  single,  short,  rounded  prolongation,  called  the  papilla, 
at  the  end  of  which,  with  a  low  magnifying  power,  several  tiny  open- 
ings may  be  seen.  The  cut  surface  of  the  papilla  presents  a  uniform 
grayish  appearance,  while  the  segment  of  the  medulla  adjacent  to  the 
cortex  presents  distinct  bands  or  striae,  radiating  from  the  papilla  and 
extending,  in  the  form  of  narrow,  isolated,  tapering  rays,  into  the  cor- 
tex, almost  to  the  surface  of  the  organ ;  these  cortical  rays  are  called 
medullary  rays.  The  cortex  surrounding  the  medulla  like  a  thick 
shell,  and  covered  by  a  firm,  dense  layer  of  connective  tissue — the 
capsule — consists,  besides  the  medullary  rays,  of  a  grayish  granular 
substance  lying  in  the  form  of  elongated  truncated  pyramids  between 
the  rays,  and  in  a  thin  irregular  layer  of  the  same  appearance  directly 
beneath  the  capsule.  The  grayish  pyramidal  portions  of  the  cortex 
are  called  cortical  pyramids,  and  the  entire  substance  of  the  cortex, 
exclusive  of  the  medullary  rays,  is  sometimes  called  the  labyrinth.  If 
the  blood-vessels  of  the  kidney  have  been  injected  with  some  colored 
substance,  or  if  the  vessels  are  well  filled  with  blood,  tiny  vessels  can 
be  seen  passing  off  from  the  large  trunks  which  lie  between  the  cortex 
and  medulla  and  extending  radially  toward  the  surface  of  the  organ 
through  the  center  of  the  cortical  pyramids,  and  at  each  side  of  these 
vessels  may  be  seen  a  row  of  minute  globular  structures,  which  are  the 
Malpighian  bodies  or  glomeruli. 

The  human  kidney  differs  from  that  which  we  have  just  described, 
in  that  it  consists  of  a  number  of  just  such  structures  crowded  together 
to  form  a  single  organ.  In  embryonic  life  its  composite  nature  is  evi- 
dent, because  each  renculus — as  each  portion  corresponding  to  the 
rabbit's  kidney  is  called — is  separated  from  its  neighbors  by  a  certain 
amount  of  connective  tissue,  giving  the  surface  of  the  organ  a  lobu- 
lated  appearance.  As  the  individual  matures,  the  renculi  usually 
become  merged  into  one  another,  so  that  we  no  longer  see  on  the  sur- 
face any  trace  of  its  composite  character.  This  is  betrayed,  however, 
by  the  fact  that  the  medullary  pyramids  of  the  primitive  renculi  per- 


g6  NORMAL  HISTOLOGY. 

sist  as  separate  structures,  and  we  thus  have  in  the  adult  human  kid- 
ney just  as  many  medullary  pyramids  and  papillae  as  there  were  original 
renculi.  We  find  also,  which  is  a  striking  feature  in  the  adult  human 
kidney,  that  the  cortical  substance  is  not  confined  to  the  cortex,  but 
extends  into  the  pelvis  between  the  papillae,  often  as  far  as,  and  some- 
times farther  than  the  papillae  themselves.  Not  very  infrequently  the 
division  between  the  primitive  renculi  are  not  entirely  obliterated  in 
the  process  of  development,  and  the  surface  of  the  kidney,  even  in  the 
adult,  is  distinctly  lobulated. 

Having  thus  acquainted  ourselves  with  the  general  structure  of  the 
kidney,  we  have  now  to  study  the  elements  of  which  it  is  composed, 
and  the  way  in  which  they  are  grouped  to  form  the  different  parts 
above  described.  The  kidney  is  a  tubular  gland ;  the  innumerable 
tubes  of  which  it  is  mainly  composed  are  lined  with  cells,  differ  greatly 
in  form  and  diameter  in  different  parts,  and  run  a  very  tortuous  course 
from  their  origin  in  the  cortex  to  their  termination  in  the  tiny  open- 
ings, above  mentioned,  at  the  apex  of  the  papillae.  They  constitute, 
with  the  walls  of  the  glomeruli,  in  which  they  originate,  the  paren- 
chyma  of  the  kidney.  In  addition  to  the  parenchyma  we  have  then  to 
study  the  connective  tissue  or  interstitial  tissue  of  the  organ  and  the 
blood-vessels. 

The  tubes  of  the  kidney,  called  in  general  uriniferous  tubules,  con- 
sist of  an  apparently  homogeneous  membrana  propria  lined  throughout 
with  a  single  layer  of  epithelial  cells,  which  differ  greatly  in  character 
and  form  in  different  parts  of  the  tubules ;  the  tubules  have  received 
special  names  in  different  parts  of  their  course.  Each  tubule  com- 
mences within  a  cortical  pyramid,  in  a  glomerulus  with  whose  capsule, 
presently  to  be  described,  its  membrana  propria  is  continuous  ;  it  is 
narrow  as  it  leaves  the  glomerulus,  but  broadens  out  at  once  into  a 
wide,  convoluted  canal — convoluted  tubule — which  winds  about  in  the 
pyramid,  finally  approaching  a  medullary  ray;  this  it  enters,  and,  sud- 
denly becoming  very  narrow,  descends  more  or  less  deeply  into  the 
medulla;  here,  widening  somewhat,  it  turns  sharply  on  itself  and 
ascends  into  a  medullary  ray  again.  This  portion  of  the  tube,  from 
the  point  at  which  it  becomes  narrow  and  begins  to  descend  in  the 
medullary  ray,  is  called  HenWs  loop,  and  the  arms  of  the  loop  are 
called,  following  the  course  which  the  description  has  taken,  the 
descending  or  narrow,  and  the  ascending  or  broad  arms  of  Henle's 
loop.  The  ascending  arm  of  Henle's  loop,  on  arriving  in  the  cortex, 
widens  and  enters  a  cortical  pyramid,  forming  what  is  known  as  the 
intercalated  tubule ;  this  resembles  the  convoluted  tubules,  among 
which  it  winds  in  and  out  in  the  cortical  pyramids  ;  it  is  much  shorter, 
however,  and  passes  over,  entering  a  medullary  ray  into  a  straight 
uriniferous  tubule.  This  is  again  narrower  than  the  convoluted  tubules, 
and  passes  directly  downward  through  the  medulla,  joining  other  simi- 
lar tubules  dichotomously  and  becoming  larger  as  it  does  so,  until  at 
length  it  opens  at  the  apex  of  a  papilla.  The  uriniferous  tubules  run 
an  entirely  independent  course  until,  as  straight  tubules,  they  join  one 
another  in  twos  to  form  the  outlet  ducts. 

Vvre  have  now  to  consider  the  epithelium  which  lines  the  tubules. 


THE   KIDNEY.  97 

Commencing  at  the  straight  tubules,  we  find  that  in  their  lower  portion 
they  are  lined  by  cylindrical  cells  with  large  nuclei  and  transparent 
cell-bodies;  farther  up  the  cells  become  more  nearly  cuboidal,  and 
are  often  flattened  in  the  medullary  rays.  The  epithelium  of  the  con- 
voluted and  intercalated  tubules  is  nearly  similar  in  character,  and 
consists  of  large,  granular,  nucleated  cells  nearly  filling  the  tube,  whose 
outlines  are  not  usually  well  defined,  and  which  present  when  fresh  or 
well  preserved  a  series  of  parallel  striae  in  the  protoplasm.  By  most 
of  the  common  modes  of  preparation,  however,  the  cells  look  simply 
like  a  granular  mass  in  which  nuclei  are  embedded,  and  which  is  often 
divided  by  fissures  into  irregular-shaped  pieces.  The  ascending  arm 
of  Henle's  loop  is  lined  with  pyramidal  or  low,  cylindrical,  granular 
cells  ;  while  the  very  narrow  descending  arm  is  lined  with  flat,  trans- 
parent cells,  whose  nuclei  usually  project  into  the  lumen  of  the  tube. 

The  glomeruli  consist,  in  the  first  place,  of  a  membranous,  appar- 
ently structureless  capsule,  similar  to  and  continuous  with  the  membrana 
propria  of  the  tubules.  The  epithelium  of  the  convoluted  tubules,  as 
the  latter  join  the  glomeruli,  becomes  flattened  and  continuous  with  a 
layer  of  very  thin  transparent  cells,  which  completely  line  the  capsule. 
At  one  side  of  the  glomerulus,  usually  opposite  to  the  attachment  of 
the  tubule,  a  small  artery — the  vas  afferens — pierces  the  capsule  and 
immediately  divides  into  a  number  of  capillary  loops,  which  wind 
about  one  another,  forming  a  complicated  vascular  tuft ;  the  blood 
from  this  tuft  is  collected  into  a  vein — vas  efferens — which  is,  as  a  rule, 
somewhat  smaller  than  the  afferent  artery,  and  leaves  the  glomerulus 
near  the  point  where  the  latter  enters.  The  capillary  tuft  within  the 
glomerulus  is  covered  also  with  a  layer  of  flat  cells  like  those  lining  the 
capsule. 

Let  us  now  review  the  positions  of  the  different  parts  of  the  tubules 
in  the  kidney.  In  the  papillae  are  the  termini  of  the  straight  tubules  ; 
in  the  medulla  lie  the  straight  tubules  and  portions  of  Henle's  loops  ; 
in  the  medullary  rays  are  the  upper  portions  of  the  straight  tubules, 
and  of  both  arms  of  Henle's  loops  ;  in  the  labyrinth,  including  the 
cortical  pyramids,  are  the  convoluted  tubules,  intercalated  tubes,  and 
the  glomeruli. 

The  distribution  of  the  blood  in  the  kidney  yet  remains  to  be  con- 
sidered. The  larger  branches  of  the  renal  arteries,  accompanied  by 
the  veins,  enter  the  organ  at  the  bases  of  the  medullary  pyramids,  and 
divide  into  large  arching  trunks  which  pass  in  various  directions,  with 
their  convexity  toward  the  cortex,  along  the  irregular  boundary  line 
between  the  cortex  and  medulla.  From  the  convex  side  of  the  arte- 
rial trunks  spring  numerous  small  branches,  each  of  which  enters  at 
once  the  apex  of  a  cortical  pyramid,  and  proceeds  directly  toward  the 
surface  of  the  organ  ;  these  arteries  are  called  interlobular  arteries* 

*  If  we  consider  a  circumscribed  portion  of  the  cortex  of  the  kidney,  having  for  its 
centre  a  medullary  ray,  and  extending  on  every  side  as  far  as  to  the  nearest  interlobu- 
lar vessels,  we  see  that  in  this  limited  area  we  have  all  of  the  essential  structural  ele- 
ments of  the  cortex — a  medullary  ray  surrounded  by  convoluted  tubules  and  glome- 
ruli. Such  groups  of  elements,  although  they  have  by  no  means  a  separate  existence, 
and  can  be  but  indefinitely  bounded,  have  been  called  lobuli  ;  and  hence,  the  vessels 
passing  between  them  are  termed  interlobular  vessels. 

7 


98  NORMAL  HISTOLOGY. 

From  these  interlobular  arteries  lateral  twigs  are  given  off  at  frequent 
intervals,  which,  after  passing  a  short  distance,  enter  the  glomeruli  as 
the  arteriae  afferentes.  On  leaving  the  glomerulus,  the  efferent  vein — 
which  still  carries  arterial  blood — breaks  up  into  a  capillary  net-work 
which  lies  chiefly  in  the  medullary  rays,  and  whose  meshes  are  narrow 
and  elongated,  corresponding  to  the  straight,  narrow  tubules,  which  it 
supplies  with  blood.  From  this  long-meshed  net-work  the  blood 
returns  to  the  labyrinth  again,  where  it  is  distributed  in  wider  capilla- 
ries through  another  net-work  whose  meshes  are  rounded,  corresponding 
to  the  convolutions  of  the  tubules  in  this  region.  It  is  then  collected 
into  small  veins,  which  in  turn  pour  it  into  interlobular  veins,  and 
these,  following  the  course  of  the  interlobular  arteries,  finally  pour  it 
into  large,  arched  trunks  at  the  base  of  the  pyramids.  Although,  for 
the  most  part,  the  course  of  the  blood  after  leaving  the  glomeruli  is  as 
above  described,  a  certain  portion  of  it  in  some  cases  enters  the  capil- 
laries around  the  convoluted  tubules  at  once,  before  or  without  passing 
outward  to  a  medullary  ray.  At  the  surface  of  the  kidney,  just  beneath 
the  cortex,  the  smaller  venous  trunks  may  not  infrequently  be  seen 
centering  in  the  commencement  of  an  interlobular  vein,  thus  forming 
the  well  known  Stellultz  VerheyeniL  The  medulla  receives  its  blood 
in  part  from  the  vasa  efferentes  of  the  glomeruli,  which  lie  near  the 
boundary,  between  cortex  and  medulla ;  in  greater  quantity  from  the 
large  arterial  arches.  Vessels  from  the  latter  sources  descend  in 
spreading  tufts  called  vasa  recta,  between  the  tubules,  and  break  up 
into  a  long-meshed  capillary  net,  the  blood  from  which  is  collected  in 
part  into  a  round-meshed  venous  net-work  in  the  papillae,  and  returned 
in  straight  veins  along  the  tubules  to  the  venous  arches,  and  in  part 
passes  directly  back  to  the  arches  without  going  down  to  the  papillae. 
The  capsule  of  the  kidney  receives  its  blood  in  part  from  the  terminal 
twiglets  of  the  interlobular  arteries,  in  part  from  terminal  branches  of 
the  phrenic,  lumbar,  and  suprarenal  arteries. 

The  connective  tissue  of  the  kidney,  aside  from  that  which  forms 
the  capsule  and  pelvis,  and  is  distributed  along  the  walls  of  the  larger 
blood-vessels,  or  found  in  the  walls  of  the  blood-vessels  themselves, 
or  in  the  tunicae  propriae  of  the  tubules,  or  the  capsules  of  the  glom- 
eruli, is  very  small  in  quantity.  It  may,  however,  be  demonstrated 
here  and  there,  and  is  most  abundant  in  the  vicinity  of  the  papillae. 

PRACTICAL    STUDY. 

Rabbit's  Kid?iey — General  View. — A  fresh  rabbit's  kidney  is  cut 
transversely  across  the  middle,  and  placed  in  a  two  per  cent,  solution 
of  bichromate  of  potassium,  where  it  remains  for  ten  days ;  it  is  then 
washed  and  the  hardening  completed  with  alcohol.  Thin  sections 
are  made  across  the  entire  organ,  including  cortex,  medulla,  and 
papilla,  stained  with  haematoxylin  and  eosin,  and  mounted  in  Canada 
balsam. 

Isolated  Tubules. — A  small  fragment  of  kidney,  including  both 
cortical  and  medullary  portions,  is  placed  in  a  mixture  of  equal  parts 
of  alcohol  and  strong  hydrochloric  acid  ;   the  acid  partially  dissolves 


THE   KIDNEY.  99 

the  intertubular  tissue,  so  that  after  a  time  the  tubules  can  easily  be 
pulled  apart.  This  is  usually  effected  in  about  twelve  hours  ;  but  the 
specimen  should  be  examined  from  time  to  time  after  it  has  been  in 
the  acid  for  eight  hours,  and  removed  as  soon  as  the  object  is  accom- 
plished. It  is  allowed  to  soak  for  a  few  hours  in  water  to  remove 
the  excess  of  acid,  and  then  small  bits  are  torn  off,  including  cortical 
and  medullary  substance,  and  the  tubules  carefully  separated  on  a 
slide.  They  are  extremely  brittle,  and  great  care  must  be  used  in 
their  isolation  :  this  may  be  done  by  needles,  or  by  Rindfleisch's  dis- 
sociation method  (see  page  128)  ;  or  better,  by  allowing  small  drops 
of  water  to  fall  upon  the  specimen  from  a  pipette.  When  the  disso- 
ciation is  partially  effected,  a  drop  of  picric  acid  should  be  added, 
and  the  specimen  mounted  and  preserved  in  it. 

Sections  of  Uninfected  Kidney. — A  perfectly  fresh  kidney  should 
be  cut  into  small  pieces,  and  hardened  in  Muller's  fluid  and  alcohol. 
Sections  are  to  be  made  in  a  plane  vertical  to  the  surface  of  the  or- 
gan, including  the  cortical  and  medullary  portions,  and  also  in  a  plane 
parallel  to  the  surface,  through  the  cortical  portion.  The  sections 
are  stained  with  hematoxylin,  and  mounted  in  glycerine  ;  or  stained 
with  hematoxylin  and  eosin,  and  mounted  in  balsam. 

Sections  of  Injected  Kidney. — These  should  be  made  in  the  same 
directions  as  the  last,  and  mounted  in  balsam. 


CHAPTER    XV. 

THE    GENERATIVE    ORGANS. 

MALE  GENERATIVE  ORGANS. 

We  shall  confine  our  study  of  these  organs  to  the  testicle,  prostate 
gland,  and  parts  of  the  penis. 

The  Testicle. 

This  is  a  tubular  gland,  whose  chief  specific  secretion  is  the  sper- 
matozoa ;  it  is  enclosed  by  a  firm,  dense  connective-tissue  capsule, 
called  the  albuginea,  which  sends  inward  several  incomplete  septa, 
which  divide  the  organ  into  a  number  of  communicating  cavities,  and 
unite  at  its  posterior  superior  part  in  a  dense,  wedge-shaped  mass  of 
connective  tissue,  called  the  corpus  Highmori.  The  more  or  less 
conical  cavities  between  the  septa  contain  winding,  anastomosing,  and 
looped  tubules,  called  seminiferous  tubules,  which  constitute  the 
secreting  portion  of  the  organ.  As  the  tubules  approach  the  corpus 
Highmori,  they  become  narrower  and  straighter — tubuli  recti— and 
are  lined  with  cylindrical  epithelium,  and,  on  entering  that  body,  form 
a  net-like  series  of  intercommunicating  channels  lined  with  flattened 
cells,  called  the  rete  testis.  The  channels  of  the  rete  testis  are  con- 
tinuous with  several  tubules,  which,  passing  upward  and  backward, 
become  very  much  convoluted,  and  form  a  number  of  conical  masses 
— coni  vasculosi — which  largely  constitute  the  head  of  the  epididymis. 
The  tubules  of  the  coni  vasculosi  gradually  unite  as  they  descend  to 
form  a  single  canal,  which,  with  numerous  windings  and  contortions, 
constitutes  the  body  and  tail  of  the  epididymis,  and  finally  becomes 
continuous  with  a  straight,  thick-walled,  ascending  tube — the  vas 
deferens. 

The  seminiferous  tubules,  which  chiefly  concern  us  here,  lie  em- 
bedded in  loose,  delicate  connective  tissue,  which  is  abundantly  sup- 
plied with  blood-vessels,  and  contains  many  cells  ;  among  the  ordi- 
nary connective-tissue  cells  of  various  forms,  large  granular,  often 
pigmented  cells  are  not  infrequently  seen  lying  singly  or  in  groups, 
and  sometimes  in  rows  along  the  blood-vessels  ;  their  nature  and  sig- 
nificance are  still  doubtful.  The  tubules  have  a  distinct  membrana 
propria,  formed  of  several  layers  of  thin,  transparent  cells,  closely 
packed  together  to  form  a  laminated  membrane  ;  they  are  lined  with 
several  layers  of  cells  piled  irregularly  over  one  another,  which  differ 
in  form  under  different  circumstances,  and    sometimes    in    different 


THE   GENERATIVE   ORGANS.  IOI 

parts  of  the  same  gland.  In  a  tubule  which  is  not  producing  sperma- 
tozoa, the  outer  row  of  cells — those  lying  upon  the  membrana  propria 
— are  large,  granular,  well-defined,  nucleated  cells  ;  and  upon  these  lie 
two  or  three  irregular  layers  of  smaller  nucleated  cells  with  ill-defined 
cell-bodies.  Between  the  cells  of  the  inner  layers  a  peculiar,  irregular 
branching  net-work  is  seen,  which,  by  most  recent  observers,  is  be- 
lieved to  be  formed,  for  the  most  part,  by  branches  of  the  outer  row 
of  cells ;  and  it  is  supposed  that  it  serves  as  a  loose  frame-work,  in 
which  the  inner  cells  are  supported.  The  cavity  of  these  tubules  may 
be  filled  with  a  more  or  less  granular  fluid,  or  with  detritus,  or  may 
contain  mature  spermatozoa. 

The  mature  spermatozoa  differ  in  form  in  different  animals  ;  in 
man  they  consist  of  a  flattened,  pear-shaped  portion,  called  the  head, 
the  small  end  of  which  is  directed  forward ;  and  a  delicate,  tapering, 
almost  filiform  portion,  called  the  tail ;  while  between  the  head  and 
tail  is  a  short,  narrow  segment,  called  the  middle  piece.  When  living, 
and  under  favorable  conditions,  the  spermatozoa  are  capable  of  per- 
forming rapid  movements,  the  whole  organism  being  driven  hither  and 
thither  by  wavy  vibrations  of  the  tail. 

In  tubules  which  are  in  full  functional  activity,  the  cavity  is  usually 
filled  with  more  or  less  granular  material  and  mature  and  immature 
spermatozoa,  and  on  every  side  a  multitude  of  clusters  or  bundles  of 
developing  spermatozoa  are  seen,  with  the  heads  embedded  among  the 
cells  of  the  inner  layers,  and  the  tails  stretching  brush-like  into  the 
cavity  of  the  tube.  According  to  the  most  recent  observations,  the 
spermatozoa  are  formed  from  certain  of  the  larger  and  smaller  cells  of 
the  inner  layers,  which  are  called  spermatoblasts.  The  process  of  de- 
velopment according  to  this  view  commences  in  the  nuclei  of  the 
spermatoblasts,  which  become  converted  into  the  body,  while  the  tail 
is  an  outgrowth  from  the  nucleus,  or  is  produced  by  a  transformation 
of  a  portion  of  the  cell-protoplasm.  Others  believe  that  the  sperma- 
tozoa are  formed  by  the  growth  inward,  from  the  large  outer  cells,  of  a 
long-stemmed  bud-like  process,  whose  dilated  end  divides  into  a  num- 
ber of  longitudinal  segments,  each  of  which  finally  becomes  a  sperma- 
tozoon. 

PRACTICAL   STUDY. 

Living  Spermatozoa. — A  cut  is  made  into  the  head  of  the  epididy- 
mis of  the  testicle  of  a  recently  killed  buck  rabbit  or  guinea-pig,  and 
a  drop  of  the  milky  fluid  which  exudes  is  received  on  a  slide  and  diluted 
with  one-half  per  cent,  salt  solution.  The  heads  of  the  organisms  in 
the  rabbit  and  guinea-pig  are  larger  than  in  man,  and  ovoid;  and  not 
infrequently  in  such  a  specimen  immature  forms  may  be  seen.  After 
the  movements  have  been  sufficiently  studied,  water  is  allowed  to  run 
under  the  cover-glass,  when  the  movements  will  presently  cease. 

Sections  of  Testicle. — Small  pieces  of  the  human  testicle,  as  fresh  as 
possible,  or  from  the  testicle  of  the  cat  or  rabbit,  are  placed  in  two  per 
cent,  solution  of  bichromate  of  potash,  and  left  for  ten  days,  the  har- 
dening being  completed  in  alcohol.  Sections  from  different  parts  are 
stained  with  hematoxylin  and  eosin,  and  mounted  in  glycerine. 


102  NORMAL   HISTOLOGY. 


The  Prostate  Gland. 


The  prostate  is  a  racemose  gland,  in  which  the  alveoli,  instead  of 
being  more  or  less  spheroidal,  as  is  usually  the  case  in  racemose  glands, 
are  often  very  much  elongated  and  irregular  in  shape,  and  very  fre- 
quently present  high,  narrow,  irregular  folds  in  their  walls.  The  alveoli 
are  lined  for  the  most  part  with  a  single  layer  of  cylindrical  epithelium. 
The  excretory  ducts  are  lined,  within  the  gland,  with  cylindrical,  which 
pass  over  into  flattened  cells  as  they  approach  the  urethral  orifice.  The 
alveoli  and  ducts  lie  embedded  in  a  dense  mass  of  interlacing  bundles 
of  smooth  muscular  tissue,  intermingled  with  elastic  fibres  and  a  small 
amount  of  fibrillar  connective  tissue.  In  the  periphery  of  the  gland 
the  muscular  tissue  forms  a  sort  of  capsule  of  varying  thickness,  which 
in  turn  is  enclosed  in  a  fibrous  tunic. 

PRACTICAL  STUDY. 

Section  of  Gland. — A  bit  of  that  portion  of  the  prostate  gland 
which  lies  behind  the  urethra  is  placed  in  two  per  cent,  solution  of 
bichromate  of  potash,  and  after  a  week  transferred  to  alcohol ;  when 
sufficiently  hard,  sections  are  made  and  stained  with  hematoxylin  and 
eosin,  and  mounted  in  balsam. 

Urethra  and  Corpus  Spongiosum. 

The  tissues  which  form  the  penis  are  so  various,  and  present  such 
essential  differences  in  their  arrangement  in  different  parts  of  the  or- 
gan, that  a  detailed  study  of  its  structure  does  not  lie  within  the  scope 
of  these  lessons.  Inasmuch,  however,  as  the  urethra  is  so  frequently 
the  seat  of  surgical  operations,  and  as  the  corpus  spongiosum,  which 
surrounds  a  portion  of  it,  presents  us  an  example  of  a  variety  of  tissue 
which  we  shall  have  no  opportunity  to  study  elsewhere  in  these  les- 
sons, it  is  desirable  to  briefly  consider  their  structure  here. 

The  urethra,  divided  into  three  portions — a  prostatic,  a  membranous, 
and  a  spongy — consists  of  a  mucous  membrane  which  is  surrounded 
by  a  muscular  sheath.  The  considerable  differences  in  structure  which 
its  different  parts  present  are  due  chiefly  to  variations  in  the  form  of 
the  epithelium  and  the  character  and  arrangement  of  the  muscular  tis- 
sue. The  mucosa  is  composed,  in  all  parts  alike,  of  fibrillar  connec- 
tive tissue  containing  numerous  elastic  fibres  and  richly  furnished  with 
cells  ;  upon  the  free  surface  of  this  rests  the  epithelium,  which  at  the 
meatus  and  in  the  fossa  navicularis  is  of  the  flat,  laminated  variety ; 
this  passes  over  into  a  single  row  of  cylindrical  cells  which  line  the 
spongy  portion  ;  and  this  in  turn  merges  into  the  spheroidal,  pear- 
shaped,  prismatic  and  flattened  cells  of  the  prostatic  portion,  which, 
lying  in  several  layers,  are  quite  similar  in  form  and  arrangement  to 
the  epithelium  of  the  bladder.  Outside  the  mucosa,  and  but  indis- 
tinctly separated  from  it,  is  the  submucosa,  which  consists  of  connec- 
tive tissue  with  elastic  fibres,  and  is  especially  characterized  by  a 
dense  net-work  of  veins  which  receive  their  blood  from  the  rich  capil- 


THE    GENERATIVE   ORGANS.  IO3 

lary  system  lying  in  the  mucosa  beneath  the  epithelium.  This  venous 
net-work  is  so  dense  and  the  vessels  so  large  as  to  lend  to  the  sub- 
mucosa,  especially  in  the  prostatic  and  membranous  portions,  the 
character  of  erectile  tissue  (see  below).  Here  and  there,  in  the 
spongy  portion,  larger  and  smaller  irregular  depressions,  called 
lacuna  Aforgagni,  are  seen  in  the  surface  of  the  mucous  membrane. 
Racemose  glands,  called  Littre 's  glands,  lie  embedded  in  the  mucous 
membrane,  sometimes  extending  into  the  muscular  tunic  ;  their  ex- 
cretory ducts,  sometimes  short,  sometimes  long  and  tortuous,  open  on 
the  surface  of  the  mucous  membrane.  In  the  prostatic  portion  the 
prostatic  and  ejaculatory  ducts  pierce  the  mucous  membrane,  as  do 
the  ducts  of  Cowper's  glands  that  of  the  posterior  segment  of  the 
spongy  portion.  The  muscular  tunic  of  the  urethra  consists,  in  gen- 
eral, of  an  inner  longitudinal  and  an  outer  circular  layer  of  smooth 
muscle-cells,  but  varies  greatly  in  structure  in  the  different  portions. 
The  outer  layers  of  the  posterior  portion  of  the  canal  are  formed  in 
part  of  striated  muscle.  The  musculosa  of  the  spongy  portion  con- 
sists entirely  of  smooth  muscle-cells  and  forms  a  complete  circular 
layer  in  the  posterior  region  alone,  while  anteriorly  only  scattered 
transversely  and  obliquely  placed  cells  are  found. 

The  urethra  is  enclosed  through  a  part  of  its  length  in  the  corpus 
spongiosum,  which  is  largely  composed  of  a  kind  of  tissue  called 
erectile  tissue;  and  before  describing  this  bodv  it  will  be  well  to  con- 
sider  for  a  moment  the  nature  of  this  kind  of  tissue.  Erectile  tissue 
in  certain  cases  consists  simply  of  a  somewhat  circumscribed  collec- 
tion of  larger  and  smaller  veins,  which  under  certain  circumstances 
may  become  distended  with  blood,  thus  causing  the  part  in  which 
they  lie  to  expand  ;  in  other  cases  it  consists  of  numerous  larger  and 
smaller  irregular  communicating  cavities,  separated  from  one  another 
by  broad  or  narrow  interlacing  bundles  of  connective  tissue  and 
smooth  muscular  tissue,  and  in  communication  with  arterial  trunks 
or  capillary  blood-vessels  ;  the  cavities  are  lined  with  endothelium, 
and,  while  usually  containing  but  a  small  amount  of  blood,  and  in  this 
condition  appearing  simply  as  slits  or  narrow  irregular  spaces  in  the 
tissue,  they  may,  under  certain  conditions,  become  distended  with 
blood,  when  they  assume  the  character  of  spheroidal  or  broad  elon- 
gated cavities,  and  thus  cause  a  considerable  increase  in  volume  of  the 
part  in  which  they  are  situated.  The  corpus  spongiosum  is  composed 
largely  of  erectile  tissue  of  the  character  last  described. 

The  corpus  spongiosum  is  enclosed  in  a  dense  connective-tissue 
sheath  which  contains  elastic  fibres  and  a  few  smooth  muscle-cells. 
From  this  a  multitude  of  narrow  trabecular  or  septa,  composed  of  es- 
sentially the  same  kinds  of  tissue,  pass  inward  in  various  directions, 
and  dividing  and  subdividing,  form  a  most  intricate  net-work,  whose 
meshes  are  blood-cavities  and  are  lined  with  endothelium.  This  sys- 
tem of  trabecular  and  septa  becomes  continuous  within  with  the  sub- 
mucosa  of  the  urethra.  This  erectile  tissue  is  most  abundant  below 
the  urethra ;  above,  the  blood  is  collected  into  venous  trunks,  which 
join  similar  trunks  from  the  corpora  cavernosa  above,  and  convey  the 
blood  away  from  the  organ. 


104  NORMAL  HISTOLOGY 


PRACTICAL  STUDY. 


Transverse  Sections. — The  penis  is  to  be  divided  transversely  into 
two  or  three  pieces  and  put  into  two  per  cent,  solution  of  bichromate 
of  potassium ;  after  a  week  it  is  transferred  to  alcohol.  When  the 
hardening  is  complete,  the  corpus  spongiosum,  with  the  posterior  por- 
tion of  the  urethra,  maybe  dissected  out  and  transverse  sections  made 
from  the  different  regions.  They  are  stained  with  haematoxylin  and 
eosin,  and  mounted  in  balsam. 

THE   FEMALE   GENERATIVE   ORGANS. 

The   Ovary. 

The  ovary  is  a  gland  in  which  the  glandular  elements,  in  the  form 
of  vesicular  alveoli,  have  no  excretory  ducts,  but  discharge  their  con- 
tents by  rupture  at  the  surface  of  the  organ.  These  alveoli  are  called 
Graafian  follicles,  and  their  specific  secretion  is  the  ovum.  They  lie 
embedded  in  a  connective-tissue  stroma,  the  interstitial  tissue  of  the 
organ,  and  in  the  adult,  are  confined  to  the  peripheral  zone.  We  have 
then,  in  studying  the  ovary,  to  consider  the  interstitial  tissue  with  its 
blood-vessels,   and  the  glandular  tissue  or  the  Graafia?i  follicles. 

On  looking  at  a  thin  transverse  section  of  the  ovary,  we  see  that 
it  presents  two  indistinctly  separated  zones  :  an  inner  or  central  zone, 
including  the  ramifications  of  the  larger  blood-vessels,  and  called  the 
medullary  portio?i ;  and  an  outer  cortical  zone,  in  which  the  follicles 
lie.  The  interstitial  tissue  consists  throughout  the  entire  organ  of 
connective-tissue  bundles,  which  in  the  medullary  portion  are  some- 
what loosely  interwoven  and  associated  with  elastic  fibres  and  smooth 
muscle-cells.  In  the  cortical  zone  the  muscular  elements  fail,  the 
connective-tissue  bundles  are  finer  and  cross  and  interlace  in  all 
directions,  surrounding  the  follicles,  in  whose  vicinity  they  are  modi- 
fied so  as  to  form  a  kind  of  capsule.  Near  the  surface  of  the  organ 
the  fibres  arrange  themselves  in  crossing  layers  to  form  a  dense  en- 
closing sheath  called  the  albuginea.  The  connective  tissue  of  the 
ovary,  especially  in  the  cortical  zone,  contains  a  great  number  of 
spindle-shaped,  flattened  and  irregular  stellate  branching  cells,  and 
not  infrequently  larger  and  smaller  pigmented  cells.  The  surface  of 
the  organ  is  covered  by  a  single  layer  of  cylindrical  epithelium,  whose 
extreme  significance  we  shall  recognize  when  we  study  the  develop- 
ment of  the  Graafian  follicles.  The  ovary  has  no  serous  covering,  the 
peritoneum  being  replaced  by  the  epithelial  cells.  The  interstitial 
tissue  of  the  ovary  is  very  vascular  ;  the  large  arterial  trunks,  entering 
from  the  broad  ligament,  divide  and  subdivide  in  the  medullary  por- 
tion and  pass  off  in  larger  and  smaller  twigs  into  the  cortex,  from 
which  an  abundant  capillary  net-work  is  formed  around  the  follicles. 
The  arteries  frequently  run  a  very  tortuous  course,  twisting  and  turn- 
ing upon  themselves,  and  are  characterized,  moreover,  by  the  great 
abundance  of  smooth  muscle-cells  in  their  walls.     Lymphatic  vessels 


THE   GENERATIVE   ORGANS.  105 

are  abundant.  Concerning  the  distribution  of  nerves,  our  knowledge 
is  still  very  meagre. 

We  have  now  to  consider  the  structure  of  the  Graafian  follicles,  to 
which  the  just  described  structures  are  subservient.  The  Graafian 
follicles  in  the  ovary  of  the  adult  female,  at  the  child-bearing  age, 
present  by  no  means  the  same  appearance  :  some  are  large  and  read- 
ily visible  to  the  naked  eye  ;  others  very  minute,  and  presenting  under 
the  microscope  an  entirely  different  structure.  These  differences  in 
structure  are  accounted  for  by  the  fact  that  certain  of  the  follicles  are 
mature  or  approaching  maturity,  while  others  are  still  undergoing 
development. 

We  will  first  consider  the  structure  of  those  which  are  mature  or 
nearly  so.  The  interstitial  tissue  of  the  ovary  arranges  itself  in  the 
immediate  vicinity  of  the  follicle,  in  the  form  of  a  tolerably  well-defined 
wall,  called  the  theca  folliculi,  in  which  it  is  easy  to  discover  an  external 
denser  layer  and  an  internal  layer  very  richly  supplied  with  cells,  and 
containing  an  abundant  capillary  net-work.  Within  the  theca  folliculi 
is  a  layer,  usually  several  cells  deep,  of  larger  and  smaller  spheroidal, 
or,  in  the  periphery,  cuboidal  epithelium,  called  follicular  epithelium, 
and  constituting  the  so-called  membrana  granulosa.  At  one  or  other 
side  the  follicular  epithelium  is  heaped  up  into  a  larger  or  smaller 
mass,  which  projects  into  the  cavity  of  the  follicle  and  contains  the 
ovum  ;  this  accumulation  of  cells  is  called  the  cumulus  proligerus 
or  germ-hill.  The  remainder  of  the  follicular  cavity  is  filled  with  a 
colorless  fluid,  in  which  not  infrequently  fine  granules  are  sus- 
pended. This  fluid  increases  in  quantity  as  the  follicle  approaches 
maturity,  and  the  pressure  occasioned  by  its  accumulation  probably 
conduces  in  no  slight  degree  to  the  final  rupture  of  the  follicle.  The 
follicular  epithelium  immediately  surrounding  the  ovum  is  cylindri- 
cal and  arranged  radially  about  it.  In  the  ovum  itself,  which  is  a  cell 
oi  the  most  highly  developed  type,  we  recognize  four  structural  ele- 
ments:  1,  a  thick  hyaline  membrane,  presenting,  with  high  powers,  a 
delicate,  radial  striation  and  called  the  zona pellucida  ;  2,  within  the  zona 
pellucida  is  the  cell-body,  consisting  of  coarsely  and  finely  granular 
protoplasm,  and  usually  called  the  vitellus  ;  3,  the  vitellus  encloses  a 
comparatively  large,  vesicular,  transparent,  and  sharply-outlined  nu- 
cleus, the  germinal  vesicle;  4,  the  germinal  vesicle,  which  is  usually 
somewhat  eccentrically  placed,  contains,  in  addition  to  a  clear  fluid, 
a  small,  dark,  often  almost  opaque  nucleolus,  the  germinative  spot. 

As  the  Graafian  follicles  mature,  they  approach  the  surface  of  the 
ovary,  often  projecting  above  it ;  the  walls  become  thinner  and  less 
vascular  at  the  projecting  side,  and  finally  burst  at  a  menstrual  period. 
The  ovum,  with  the  fluid  and  a  portion  of  the  follicular  epithelium,  is 
discharged,  and  through  the  hemorrhage  which  occurs  from  the  capil- 
laries in  the  follicular  wall,  the  cavity  is  filled  with  blood.  Changes 
now  occur  in  the  walls  and  cavity  of  the  follicle,  which  result  sooner 
or  later  in  cicatrization  and  obliteration  of  the  cavity.  These  changes 
vary  considerably,  depending  upon  whether  or  not  the  ovum  is  im- 
pregnated and  develops  ;  and  the  difference  expresses  itself  chiefly  in 
a  difference  in  size  and  persistence  of  the  mass  of  tissue  called  the 


106  NORMAL   HISTOLOGY. 

corpus  luteum,  which  is  produced  by  a  growth  of  certain  of  the  cells 
which  remain  after  the  rupture  of  the  follicle.  These  differences  can- 
not be  considered  here.  The  process  of  formation  and  disappearance 
of  the  corpus  luteum,  under  all  circumstances,  is  essentially  the  follow- 
ing :  the  blood  which  is  poured  out  into  the  cavity  passes  through 
the  same  retrogressive  metamorphosis  which  extravasated  blood  in 
any  part  of  the  body  may  undergo;  it  coagulates,  the  serum  is  ab- 
sorbed, the  red  cells  disintegrate,  and  the  coloring  matter  is  in  part 
taken  up  by  surrounding  tissues,  in  part  transformed  into  yellowish  or 
red  hematoidin  crystals,  which  in  turn  may  change  into  a  dark  brown 
or  black  pigment,  which  may  be  taken  up  by  surrounding  tissues  or 
remain  for  a  long  time  unchanged.  Hand  in  hand  with  these  changes 
in  the  extravasated  blood,  go  important  changes  in  the  follicular  epi- 
thelium which  is  left  behind,  and  in  the  cells  of  the  theca  folliculi. 
These  cells  proliferate  and  form  a  soft,  yellowish,  very  vascular  tissue, 
resembling  mucous  tissue,  which  presently  undergoes  fatty  degenera- 
tion. This  yellow  mass,  surrounding  and  enclosing  the  remains  of  the 
extravasated  blood,  constitutes  the  corpus  luteum  ;  and,  as  it  disap- 
pears, its  place  is  occupied  by  firm,  dense  connective  tissue,  which 
usually  persists  for  a  long  time  in  the  form  of  an  irregular  cicatrix, 
whose  cells  not  infrequently  still  contain  yellow  or  brown  or  black  pig- 
ment. 

In  order  to  understand  ail  the  forms  which  the  immature  Graafian 
follicles  present,  it  will  be  necessary  to  study  the  way  in  which  these 
structures  originate.  They  are  produced  from  the  cylindrical  epithe- 
lium which  covers  the  ovary,  and  is  called  germ  epithelium.  At  an 
early  period  of  life  the  connective-tissue  stroma  of  the  ovary  grows 
rapidly,  and  partially  encloses  groups  of  the  germinal  epithelial  cells, 
which  themselves  proliferate  and  dip  down  into  the  stroma  of  the 
organ,  in  the  form  of  elongated,  solid,  or  tubular  masses.  Presently 
these  groups  of  cells  become  separated  from  the  germinal  epithelium 
at  the  surface  by  a  still  further  growth  of  the  connective  tissue,  and 
appear  then  as  irregular  masses  of  variously  shaped  cells,  some  of 
which  are  larger  than  the  rest,  enclosed  on  all  sides  by  connective 
tissue.  A  still  further  growth  of  the  stroma  separates  these  masses  of 
cells  into  smaller  groups,  which  gradually  sink  farther  from  the  surface 
of  the  organ  and  become  more  widely  separated  from  one  another, 
while  new  masses  of  germ-epithelium  are  being  enclosed  at  the  sur- 
face. The  cells  which  now  lie  in  these  separated  cavities  do  not  all 
look  alike,  but  we  usually  find  that  one  of  them  is  larger  than  the 
others,  is  spheroidal,  and  occupies  the  centre  of  the  cavity,  while  the 
rest  are  more  or  less  cuboidal  and  arranged  in  a  single  layer  around 
the  wall,  closely  enclosing  the  central  cell.  This  is  the  young  Graafian 
folhcle,  and  the  central  cell  is  the  ovum  ;  but  it  presents  as  yet  no 
zona  pellucida.  Gradually  the  follicular  epithelium  increases  in  quan- 
tity, forming  several  layers,  and  the  ovum  becomes  eccentrically 
placed.  There  is  still  no  cavity,  but  this  soon  appears,  at  first  as  a 
slit  between  the  cells,  and  then  grows  wider  and  wider  as  fluid  accu- 
mulates "within  it.  Changes  in  the  interstitial  tissue  lead  to  the  for- 
mation of  the  theca  folliculi,  the  epithelium  directly  about  the  ovum 


THE   GENERATIVE   ORGANS.  107 

assumes  a  radial  arrangement,  the  zona  pellucida  is  formed,  and  we 
thus  have  the  structure  of  the  maturing  follicle,  with  which  we  are 
already  acquainted. 

PRACTICAL   STUDY. 

Sections  of  Adult  Ovary. — The  human  ovary,  or  that  of  a  recently 
killed  animal,  should  be  divided  transversely,  great  care  being  taken 
not  to  rub  the  surface,  since  the  germ-epithelium  easily  comes  off,  and 
placed  in  a  mixture  of  equal  parts  of  one-quarter  per  cent,  chromic 
acid  solution  and  alcohol.  After  a  week  it  is  to  be  transferred  to 
alcohol,  in  which  in  a  few  days  it  will  become  sufficiently  hard.  A 
half  is  embedded  in  wax,  the  sections  stained  with  hematoxylin  and 
eosin,  and  mounted  in  glycerine. 

Sections  of  Developing  Ovary. — The  ovary  of  a  foetal  or  new-born 
animal  is  hardened  and  prepared  as  above,  especial  care  being  taken 
not  to  rub  off  the  germ-epithelium,  since  the  chief  purpose  of  this 
preparation  is  to  show  the  formation  of  the  follicles. 

The  Uterus. 

The  walls  of  the  uterus  consist  of  crossing  and  interlacing  bundles 
of  smooth  muscle-cells,  with  a  small  amount  of  connective  tissue  en- 
closing them  and  binding  them  together.  The  muscular  tissue  is 
arranged  in  three  ill-defined  layers,  of  which  the  middle  is  the  thicker. 
It  may  be  said,  in  general,  that  the  bundles  of  the  inner  layer  run 
transversely  around  the  organ,  those  of  the  middle  layer  are  longitu- 
tudinal,  while  those  of  the  outer  layer  are  quite  irregular  ;  still  in  all 
the  layers  there  is  great  lack  of  uniformity  in  the  direction  of  the  cells. 
A  part  of  the  external  surface  of  the  uterus  is  covered  by  the  peritoneum, 
while  its  inner  surface  is  lined  with  a  mucous  membrane.  The  latter 
consists  of  a  frame-work  formed  of  a  delicate  net-work  of  fibres,  between 
which  lie  a  great  number  of  spheroidal,  fusiform,  and  branched  cells, 
and  covered  on  the  free  surface  by  cylindrical  ciliated  cells.  In  the 
mucosa  the  simple  or  branched  tubular  uterine  glands  are  embedded  ; 
they  are  often  tortuous,  and,  like  the  surface  of  the  mucous  membrane, 
are  lined  with  cylindrical  ciliated  epithelium.  The  surface  of  the  mu- 
cous membrane  of  the  body  of  the  uterus  is  smooth,  but  in  the  cervix 
it  presents  regular  folds,  the  so-called  pliccz  palmatce  ;  the  connective- 
tissue  frame-work  of  the  cervical  mucous  membrane  is,  moreover, 
firmer  in  texture,  contains  fewer  glands,  and  these,  for  the  most  part, 
are  more  or  less  globular  and  lined  with  short  cylindrical  or  cuboidal 
epithelium.  The  cylindrical  ciliated  epithelium  of  the  body  extends 
over  on  to  the  mucous  membrane  of  the  cervix,  where  it  becomes 
continuous  with  the  laminated  epithelium  covering  the  lower  portion 
of  the  canal  and  the  portio  vaginalis.  The  uterus  is  a  very  vascular 
organ ;  the  mucous  membrane  is  supplied  with  a  rich  capillary  plexus, 
which  passes  inward  close  beneath  the  surface-epithelium. 

During  menstruation  the  mucous  membrane  becomes  thickened — 
partly  owing  to  the  engorgement  of  its  blood-vessels,  and  partly  to 
the  accumulation  of  fluid  and  lymph-cells  in  its  interstices.  The 
uterine  glands  are  enlarged,  and  their  epithelium,  as  well  as  that  of  the 


108  NORMAL   HISTOLOGY. 

general  surface  of  the  cavity,  is  swollen.  To  what  extent  the  blood, 
which,  mixed  with  mucous  and  separated  epithelium,  fills  the  cavity  at 
the  menstrual  period,  is  due  to  the  rupture  of  the  engorged  capillaries, 
and  to  what  extent  to  diapedesis,  is  not  yet  definitely  determined. 

PRACTICAL    STUDY. 

Sections  of  Human  Uterus. — The  human  uterus  is  cut  into  several 
pieces  and  placed  in  two  per  cent,  solution  of  bichromate  of  potassium, 
and  after  ten  days  transferred  to  alcohol.  The  organ  should  be  pro- 
cured as  soon  as  possible  after  death,  since  the  cilia  of  the  lining 
epithelium  are  easily  destroyed  by  the  decomposition  which  commences 
very  early  in  the  uterine  cavity.  Sections  perpendicular  to  the  surface 
of  the  mucous  membrane  are  made  from  the  cervix ;  and  in  the  body, 
from  the  lower  portions  or  from  the  vicinity  of  the  entrance  of  the 
Fallopian  tubes,  since  here  the  uterine  glands  are  more  uniformly 
arranged  at  right  angles  to  the  surface.  Sections  are  stained  with 
haematoxylin  and  eosin,  and  mounted  in  Canada  balsam. 

The  Vagina. 

In  the  walls  of  the  vagina  we  recognize  three  layers :  an  outer 
fibrous  layer,  a  middle  muscular  layer,  and  a  lining  mucous  membrane. 

The  fibrous  layer,  by  means  of  which  the  vagina  is  connected  with 
adjacent  parts,  consists  of  connective  tissue  with  elastic  fibres,  and  in 
texture  is  looser  in  its  outer,  denser  in  its  inner  portions.  The  mus- 
cular layer  consists  of  bundles  of  smooth  muscle-cells  which  present 
an  indistinct  grouping  into  an  outer  portion,  formed  of  longitudinally 
arranged  cells,  and  an  inner,  in  which  they  have  in  general  a  transverse 
arrangement.  The  mucous  membrane  consists  of  delicate  connective 
tissue,  loose  in  texture  and  containing  coarser  and  finer  elastic  fibres  ; 
it  presents  numerous  transverse  folds  and  elevations  and  is  covered  by 
laminated  epithelium,  the  cells  in  the  lower  layers  being  more  or  less 
spheroidal,  but  flat  and  scale-like  at  the  surface.  Numerous  papillae 
project  into  the  epithelium.  Venous  plexuses  are  formed  within  the 
deeper  portions  of  the  mucous  membrane  which  give  to  some  parts 
of  that  structure  the  character  of  erectile  tissue. 

Glands  have  not  been  satisfactorily  demonstrated  in  the  vaginal 
mucous  membrane. 

PRACTICAL    STUDY. 

Transverse  Sections. — A  bit  of  the  vaginal  wall  should  be  stretched 
on  cork  and  placed  in  a  mixture  of  equal  parts  of  one-fourth  per  cent, 
solution  of  chromic  acid  and  alcohol,  and  after  five  days  transferred 
to  alcohol  in  which  the  hardening  is  completed.  Sections  at  right 
angles  to  the  surface  are  stained  with  haematoxylin,  and  mounted  in 
glycerine. 

Isolated  Surface-cells. — From  a  bit  of  vagina  hardened  as  above, 
or  simply  in  alcohol,  the  cells  from  the  surface  of  the  mucous  mem- 
brane are  scraped  with  a  scalpel,  and  stained  on  a  slide  with  haematoxylin 
and  mounted  in  glycerine. 


THE  GENERATIVE  ORGANS.  109 


Mammary  Gland. 


The  mammary  gland  is  a  racemose,  and  when  fully  developed  a 
lobulated  gland,  whose  spheroidal  alveoli  are  formed  by  a  membrana 
propria  composed  of  flattened  cells,  and  lined  with  cuboidal  epithe- 
lium. The  excretory  ducts,  which  in  each  gland  are  fifteen  to  twenty 
in  number,  and  open  at  the  surface  of  the  nipple,  are  lined  with  cylin- 
drical epithelium.  Fibrillar  connective  tissue  with  elastic  fibres  lies 
between  the  alveoli  and  lobules,  and  is  abundantly  furnished  with 
blood-vessels.  The  gland  presents,  both  macroscopically  and  micro- 
scopically, quite  marked  differences  in  appearance  at  different  times, 
depending  upon  age,  sex,  and,  in  the  adult  female,  upon  whether  or 
not  the  individual  is  pregnant,  and  upon  the  period  of  pregnancy  and 
the  occurrence  of  lactation. 

In  children,  as  in  the  adult  male,  the  gland  presents,  in  general,  a 
system  of  ramifying  ducts,  terminating  in  blind  or  in  more  or  less 
pouched  or  dilated  extremities  ;  these  lie  embedded  in  connective  tis- 
sue, which  makes  up  the  greater  part  of  the  bulk  of  the  gland.  At 
puberty  the  gland  of  the  female  undergoes  considerable  development : 
well-defined  alveoli  are  found  in  the  periphery  of  the  gland  connected 
with  the  excretory  ducts,  but  the  interstitial  tissue  is  still  more  abun- 
dant than  the  gland-tissue  proper.  Essentially  in  this  condition  the 
gland  remains  until  the  climacteric  period  if  pregnancy  does  not  occur. 
If  however,  the  individual  becomes  pregnant,  the  number  and  size  of 
the  alveoli  rapidly  increase  ;  the  ducts,  which  in  the  virgin  terminate 
only  in  dilated  extremities,  become  connected  with  extensive  groups 
of  newly-formed  alveoli;  the  gland  assumes  a  lobulated  character,  the 
connective  tissue  between  the  alveoli  is  very  much  less  abundant  in 
proportion  to  the  gland-tissue,  is  looser  in  texture  and  contains  a  great 
number  of  larger  and  smaller  cells,  and  frequently  fat-cells.  During 
lactation  the  alveoli  are  very  large,  their  epithelium  contains  fat-drop- 
lets in  considerable  number  and  fat  is  found  in  greater  or  less  quan- 
tity in  the  cavities  of  the  alveoli  and  in  the  excretory  ducts.  When 
the  secretory  activity  of  the  gland  ceases,  it  undergoes  involution,  the 
alveoli  become  smaller,  fat  ceases  to  be  produced  by  the  cells,  and  the 
interstitial  connective  tissue  becomes  proportionally  more  abundant. 

With  the  decline  of  the  reproductive  power  the  mammary  gland 
undergoes  a  marked  and  permanent  involution  :  the  terminal  alveoli 
disappear,  the  smaller  ducts  are  obliterated,  and  finally  little  is  left 
of  the  organ  excepting  the  deformed  and  collapsed  larger  ducts  and  a 
mass  of  connective  tissue. 

PRACTICAL    STUDY. 

Sections  of  Hardened  Gland. — Portions  of  the  gland  from  the  hu- 
man subject  or  from  some  of  the  lower  mammalia,  preferably  in  a 
condition  of  functional  activity,  should  be  hardened  in  bichromate  of 
potassium  and  alcohol,  in  the  usual  way,  and  the  sections  stained  with 
hematoxylin  and  eosin,  and  mounted  in  glycerine. 


CHAPTER  XVI. 

THE    CENTRAL    NERVOUS    SYSTEM. 

The  Spinal  Cord. 

The  spinal  cord  contains  nerve-ceils  and  nerve-fibres,  the  former  con- 
fined to  the  central,  the  latter  most  abundant  in  the  peripheral  por- 
tions. These  nerve-elements  lie  in  the  meshes  of  peculiarly  arranged 
connective  tissue  in  which  blood-vessels  ramify,  and  the  whole  is 
surrounded  by  a  vascular  connective-tissue  membrane — the  pia  mater 
spinalis.  On  its  anterior  and  posterior  surfaces,  narrow  fissures,  reach- 
ing nearly  to  the  centre,  divide  the  cord  into  lateral  halves ;  into  these 
fissures,  called  the  anterior  and  posterior  longitudinal  fissures,  the  pia 
mater  sends  a  membranous  prolongation,  the  anterior  fissure  is  com- 
plete, the  pia  being  found  on  both  of  its  sices,  which  can  be  easily 
separated  ;  while  in  the  posterior  fissure  the  sides  are  bound  together 
by  the  pia.  In  transverse  sections  of  the  cord  a  central  gray  portion 
is  seen  which  is  surrounded  by  an  irregular  white  zone.  The  form 
which  the  gray  matter  assumes  differs  considerably  in  different  parts 
of  the  cord,  but  has  in  general  the  form  of  an  unsymmetrical  H  ;  the 
cross-arm  of  the  H  is  formed  in,  great  part,  by  the  nerve-substance 
which  connects  the  lateral  halves  of  the  cord,  and  is  called  the  gray 
commissure  ;  the  uprights  of  the  H  lie  completely  embedded  within 
the  white  matter  of  the  lateral  halves  ;  the  anterior  and  broader  ends 
being  called  the  anterior  cornua,  the  posterior  and  narrow  ends  the 
posterior  cornua.  Within  the  gray  commissure,  and  separating  it  into 
an  anterior  and  a  posterior  portion,  a  narrow  canal,  called  the  central 
canal,  runs  the  entire  length  of  the  cord  :  it  is  lined,  in  early  life  at 
least,  with  cylindrical  ciliated  cells,  and  is  surrounded  by  delicate  con- 
nective tissue.  From  the  anterior  and  posterior  cornua  the  spinal 
nerves  pass  off,  dividing  the  white  matter  into  three  tolerably  distinct 
portions  called  the  anterior,  lateral,  and  posterior  columns. 

In  the  white  substance  of  the  cord  we  find  nerve-fibres  and  con- 
nective tissue  and  blood-  and  lymph-vessels.  The  fibres  are  for  the 
most  part,  medullated,  but  have  not.  so  far  as  we  can  determine  with 
the  technical  facilities  at  present  at  our  disposal,  any  neurilemma. 
They  vary  greatly  in  diameter,  and  although  a  large  majority  of  them 
run  longitudinally  for  the  greater  part  of  their  course,  we  find  in  each 
transverse  section  a  considerable  number  which  run  in  an  oblique  or 
horizontal  direction.  Thus,  we  find  in  front  of  the  anterior  gray  com- 
missure, at  the  bottom  of  the  anterior  fissure,  a  band  of  horizontally 
arranged  fibres  running  from  one   side   to  the   other,  called  the  white 


THE   CENTRAL  NERVOUS   SYSTEM.  Ill 

commissure.  The  fibres  which  pass  out  of  the  gray  matter  to  form  the 
roots  of  the  spinal  nerves,  take  also  longitudinal  and  oblique  courses. 
The  connective  tissue  of  the  white  substance,  is  intimately  con- 
nected at  the  periphery,  with  the  pia-mater.  The  inner  layers  of  the 
latter,  consisting  of  delicate  nbrillated  and  a  few  elastic  fibres,  send  into 
the  cord  numerous  branching  septa,  which  divide  the  space  occupied  by 
the  nerve-substance  into  irregular  spaces,  in  which  the  fibres  run,  each 
one  being  surrounded  by  a  delicate  net-work  of  fibrillae,  upon  which, 
here  and  there,  variously  shaped,  for  the  most  part  flattened  cells,  are 
placed.  This  supporting  connective  tissue,  which  was  formerly  be- 
lieved to  be  composed  of  branching  cells  and  closely  allied  to  reticu- 
lar connective  tissue,  is  called  neuroglia ;  and  in  addition  to  the 
fibrillar  elements  above  described  there  seems  to  be  a  greater  or  less 
amount  of  a  soft,  homogeneous  or  finely  granular  material  lying  in  the 
interstices  of  the  nerve  and  connective-tissue  fibres  ;  but  its  nature  is 
not  yet  definitely  determined.  The  amount  of  connective  tissue  in 
the  white  substance  of  the  cord  relative  to  the  nerve-fibres  varies 
considerably  in  the  different  columns ;  in  the  posterior  column  for 
example,  in  certain  parts  of  the  cord,  the  connective  tissue  is  very 
abundant,  separating  from  adjacent  parts  the  so-called  cuneiform  col- 
umns or  columns  of  Goll. 

In  the  gray  matter  of  the  cord  we  have  also  nerve  and  connective- 
tissue  elements  and  blood-  and  lymph-vessels,  the  first  consisting  of 
ganglion-cells  and  nerve-fibres.  The  nerve-fibres  are  in  part  medullated, 
in  part  naked  axis  cylinders  ;  and  besides  these,  a  multitude  of  extremely 
delicate  gray  nerve-fibrils  occur,  which  seem  partly  to  come  from  the 
breaking  up  of  axis  cylinders,  and  partly  to  be  the  delicate  branching 
processes  of  ganglion-cells.  The  nerve-cells  of  the  gray  matter  are,  for 
the  most  part,  multipolar,  and  vary  greatly  in  size,  the  largest  being 
found,  as  a  rule,  in  the  anterior  cornua.  They  are  arranged  in  irregular 
groups  in  different  parts  of  the  cord.  The  connective-tissue  elements 
of  the  gray  matter  consist  of  a  delicate  frame-work,  similar,  in  most  re- 
spects, to  that  supporting  the  nerve-fibres  between  the  coarser  septa  of 
the  white  substance.  In  the  hinder  portions  of  the  posterior  cornua 
there  is  a  circumscribed  area,  which,  in  the  fresh  cord,  has  a  peculiar 
gelatinous  appearance,  and  is  called  the  substantia  gelatinosa  of 
Rolando  ;  in  this  we  find  comparatively  few  nerve-elements  and  much 
connective  tissue,  containing  larger  and  smaller  variously  shaped 
cells.  The  limitations  of  these  lessons  will  not  permit  us  to  consider 
more  in  detail  what  is  known  of  the  course  of  the  nerve-fibres  through 
the  cord,  and  their  more  exact  relations  to  the  ganglion-cells.  The 
blood-vessels  enter  the  cord  from  the  pia,  along  the  septa,  which  the 
latter  sends  into  the  organ,  and  ramify  in  its  substance,  the  capillary 
plexuses  being  denser  in  the  gray  than  in  the  white  matter. 

TRACTICAL     STUDY. 

Transverse  Sections. — A  perfectly  fresh  human  cord  should  be 
freed  from  its  dura  mater,  short  segments  taken  from  different  portions 
of  it,  and  hardened  in  Miiller's  fluid  and  alcohol.     Thin  transverse 


112  NORMAL   HISTOLOGY. 

sections  are  made  through  the  entire  cord  in  different  portions,  and 
may  be  stained  first,  lightly  with  hematoxylin,  and  then  with  eosin  or 
carmine,  and  mounted  in  balsam.  A  very  excellent  hardening  agent 
for  the  nerve-centres  is  the  bichromate  of  ammonium,  in  which  small 
fragments  should  lie  for  a  week,  then  be  carefully  washed,  and  the 
hardening  completed  in  alcohol.  They  may  be  mounted  as  before, 
or  they  may  be  stained  first  intensely  with  aniline  green — lying  for 
twenty-four  hours  in  a  very  deeply  colored  aqueous  solution — and 
then  passed  through  alcohol  deeply  tinged  with  eosin,  and  mounted 
in  balsam.  Alcohol  dissolves  the  green  color  out  of  the  sections 
quite  rapidly,  and,  if  allowed  to  remain  in  it  too  long,  it  will  entirely 
disappear. 

The  Brain. 

In  this  organ  also  we  have  gray  and  white  matter,  but  they  are 
arranged  in  a  much  more  complicated  manner  than  in  the  spinal  cord. 
The  collections  of  gray  matter  in  the  brain  may  be  arranged,  in  a 
general  way,  in  four  distinct  groups  :  first,  as  a  direct  continuation  of 
the  gray  matter  of  the  cord,  the  gray  matter  investing  the  ventricles  ; 
second,  the  gray  matter  in  the  cerebral  ganglia  at  the  base  of  the 
brain  ;  third,  the  gray  matter  in  the  cortex  of  the  cerebrum ;  fourth, 
the  gray  matter  in  the  cerebellum  and  immediately  connected  with  it. 
These  collections  of  gray  matter  are  variously  associated  with  one 
another  by  means  of  the  nerve-fibres  of  the  white  substance.  The 
white  substance  of  both  cerebrum  and  cerebellum  consists  of  coarser 
and  finer,  but  all  very  small,  nerve-fibres,  running  in  various  direc- 
tions, and  supported  by  a  delicate  connective-tissue  frame-work,  simi- 
lar to  that  of  the  cord.  The  gray  matter  consists  here,  as  in  the  cord, 
of  ganglion-cells  and  fine  gray  fibres,  supported  by  connective  tissue. 
In  parts  of  the  gray  as  of  the  white  substance,  certain  cellular  and 
fibrous  elements  occur,  of  which  it  is  at  present  impossible  to  say 
whether  they  are  connective  or  nerve  tissue.  Indeed,  it  is  the  diffi- 
culty of  determining  the  nature  of  certain  structures  in  the  brain, 
together  with  their  extreme  delicacy  and  the  difficulty  of  isolating 
them,  which  renders  the  histology  of  the  brain  so  difficult  a  theme, 
and  explains  the  unsatisfactory  state  of  our  knowledge  concerning  it. 

We  cannot  do  more  in  these  lessons  than  to  study  briefly  the 
structure  of  two  of  the  best  known  and  at  present,  perhaps  most  inter- 
esting parts  of  the  brain — namely,  the  cortical  portions  of  the  cere- 
brum and  cerebellum. 

a.  Cortex  of  the  Cerebrum. — The  structure  of  this  part  of  the  brain 
differs  somewhat  in  different  regions,  but  that  of  one  of  the  frontal  lobes 
is  sufficiently  typical  for  our  purpose.  Here,  in  a  section  perpendicular 
to  the  surface,  and  extending  through  the  entire  depth  of  the  gray 
matter,  five  zones  or  layers  may  be  recognized,  which,  however,  merge 
gradually  into  one  another.  In  the  most  superficial  layer,  the  con- 
nective-tissue elements  preponderate,  and  among  them,  delicate 
nerve-fibrils  interlace,  and  a  few  small,  scattered,  globular,  and  elon- 
gated nerve-cells  are  found;  the  second  layer  is  characterized  by  a 


THE  CENTRAL  NERVOUS  SYSTEM.,  113 

great  number  of  small  more  or  less  pyramidal  cells  ;  the  third  and 
broadest  layer  contains  a  proportionately  smaller  number  of  ganglion- 
cells  than  the  second,  but  they  are  larger  and,  for  the  most  part, 
pyramidal,  or  broad,  spindle-shaped,  with  their  long  axes  perpendicu- 
lar to  the  surface  of  the  cortex  ;  in  the  fourth  layer,  which  is  much 
narrower  than  the  last,  are  large  numbers  of  small,  globular  and 
irregular-shaped  and  branching  cells  ;  the  fifth  layer,  finally,  contains 
medium-sized,  spindle-shaped  cells,  with  long,  tapering  processes,  to- 
gether with  a  certain  number  of  smaller,  irregular-shaped  cells.  In 
the  third  layer,  certain  of  the  delicate  nerve-fibres  begin  to  take  a 
more  regular  course  toward  the  white  matter ;  and  in  the  fourth  and 
fifth  layers,  they  are  readily  seen  in  distinct  bundles  passing  inward 
between  the  ganglion-cells. 

b.  Cortex  of  the  Cerebellum. — In  sections  through  the  cortex  of  the 
cerebellum  perpendicular  to  the  surface,  three  distinct  layers  are  rec- 
ognizable ;  the  outer,  sometimes  called  the  molecular  layer,  consists, 
like  the  outer  layer  of  the  cerebral  cortex,  of  a  delicate  connective- 
tissue  frame-work,  which  supports  fine  nerve-fibres  and  small  spindle- 
shaped  and  branching  nerve-cells  ;  the  middle,  cellular  layer,  is  char- 
acterized by  a  row  of  large  ganglion  cells,  Purkinje's  cells,  whose 
branching  processes  extend  into  and  ramify  in  the  outer  layer,  while 
the  axis-cylinder  process  (see  page  62)  passes  inward  through  the 
inner  layer ;  the  inner  granular  layer  contains  a  great  number  of 
small  spheroidal  cells  whose  nature  is  undetermined.  The  granular 
layer  merges  gradually  into  the  white  substance  and  is  thickest  at  the 
summit  of  the  convolutions,  where  also  Purkenje's  cells  are  most 
abundant.  The  blood-vessels  penetrate  the  cortex  of  both  cerebrum 
and  cerebellum,  in  the  form  of  small  arterial  twigs  from  the  pia,  and 
form  an  abundant  net-work  in  the  gray  substance,  being  somewhat 
differently  distributed  in  different  parts  of  the  organ. 

The  dura  mater  of  the  brain  is  a  dense  connective-tissue  mem- 
brane containing  numerous  elastic  fibres  and  lined  within  by  endothe- 
lial cells.  Where  it  is  attached  to  the  bones  of  the  skull,  to  whose 
inner  surface  it  acts  as  periosteum,  the  tissue  on  the  attached  surface 
is  looser  in  texture  and  abundantly  supplied  with  blood-vessels.  It 
contains  the  ordinary  flattened  connective-tissue  cells  and  usually  a 
certain  number  of  large  granular  "  plasma  "  cells.  The  dura  mater 
of  the  cord  does  not  form  the  periosteum  of  the  bones  forming  the 
spinal  canal,  and  hence  in  it  the  looser  vascular  layer  is  for  the  most 
part  wanting. 

The  Pia  Mater  of  the  brain  is  a  thin  connective-tissue  membrane, 
covered  on  its  outer  surface  with  endothelium  and  containing  an  ex- 
ceedingly abundant  net-work  of  blood-  and  lymph-vessels.  Over  the 
surface  of  the  convolutions  it  forms  a  single  membrane,  but  as  it  passes 
over  the  sulci  it  divides  into  an  inner  vascular  layer  which  dips  down 
into  the  sulci,  and  an  outer  layer  containing  but  few  vessels,  which 
bridges  over  the  space  between  adjacent  convolutions.  The  cavities  or 
spaces  thus  formed  between  the  two  layers  of  the  pia — called  subarach- 
noidal spaces — are  lined  with  endothelium,  and  are  lymph-sinuses  ; 
and  these,  as  well  as  the  numerous  lymph-channels  of  the  pia,  are  in 
8 


114  NORMAL  HISTOLOGY. 

communication  with  lymph-channels  called  perivascular  lymph-chan- 
7iels,  which  ensheath  certain  of  the  blood-vessels  as  they  enter  the  brain 
substance. 

PRACTICAL    STUDY. 

Sections  of  Cortex  of  Cerebrum  and  Cerebellum. — These  should 
be  prepared  in  the  same  way  as  the  spinal  cord  ;  pieces  from  the  cor- 
tex of  the  frontal  lobes  being  taken,  and  from  the  cortex  of  the  cere- 
bellum at  any  convenient  part ;  they  should  not  be  larger  than  a  cubic 
centimetre,  and  the  chromic  fluid  well  washed  out  before  immersion  in 
alcohol. 

Dura  Mater. — A  bit  of  this  membrane  should  be  stretched  on  a 
piece  of  cork  with  pins,  hardened  in  alcohol,  embedded  in  hardened 
liver,  and  thin  transverse  sections  made,  stained  with  hematoxylin  and 
mounted  in  glycerine. 

Pia  Mater. — A  small  piece  of  the  pia  should  be  stripped  carefully 
from  the  brain,  care  being  taken  that  none  of  the  brain  substance  ad- 
heres, stretched  on  a  cork,  hardened  in  alcohol,  stained  with  carmine 
or  picro-carmine,  and  mounted  in  glycerine.  For  the  purpose  of  study- 
ing the  endothelium,  a  small  bit  of  the  membrane  is  placed  for  forty- 
eight  hours  in  dilute  alcohol — alcohol,  i  ;  water,  2  ; — and  the  surface 
then  gently  scraped  with  a  scalpel ;  a  small  portion  of  the  separated 
cellular  mass  is  placed  on  a  slide  in  a  drop  of  glycerine  which  has 
been  colored  distinctly  red  with  eosin  and  covered  and  studied  in  the 
same. 


CHAPTER    XVII. 

THE  SKIN. 

We  recognize  in  the  skin  three  layers  of  tissue  :  i,  an  cuter,  epithe- 
lial layer,  the  epidermis;  beneath  this,  2,  a  layer  of  quite  firm  and 
dense  connective  tissue,  the  corium — true  skin,  cutis  vera,  or  derma  ; 
3,  a  layer  of  connective  tissue,  the  subcutaneous  tissue,  which,  merging 
into  the  corium  serves  to  bind  it  to  underlying  parts.  The  skin  is 
variously  modified  in  different  parts  of  the  body,  to  meet  the  different 
conditions  of  exposure  and  wear  to  which  it  is  subjected,  and  to  form 
certain  supplementary  structures  such  as  the  hair,  nails,  etc.;  and  con- 
tains various  sensory  and  secretory  structures.  In  the  epidermis  we 
recognize  two  tolerably  distinct  layers  :  1,  an  outer  or  horny  layer 
consisting  of  very  thin,  flat,  transparent,  tough,  scale-like  cells,  which 
present  for  the  most  part  no  nuclei,  and  are  packed  closely  together 
side  to  side  ;  and  2,  an  inner  layer,  the  so-called  mucous  or  Malpighian 
layer,  consisting  of  larger  and  smaller  nucleated  cells  of  varying  shape 
and  character.  In  the  deeper  portion,  adjoining  the  corium,  the  cells 
are  more  or  less  cylindrical ;  above  this  they  are  spheroidal  or  ovoidal, 
or  considerably  elongated  ;  still  nearer  the  surface  they  become  flat- 
tened, and  finally  merge  into  the  thin  cells  of  the  horny  layer.  In  the 
middle  zone,  where  the  cells  are  becoming  elongated  and  flattened, 
they  present  a  peculiar  jagged  outline,  looking  as  if  they  were  bor- 
dered by  short  delicate  spines  by  which  the  cells  seem  to  be  dove- 
tailed together.  These  spined  cells  are  very  characteristic  of  this 
part  of  the  epidermis,  and  are  also  found  in  certain  other  parts  of  the 
body  where  laminated  epithelium  occurs,  as  in  the  vagina,  mucous 
membrane  of  the  mouth,  etc.  The  relative  thickness  of  the  horny 
and  mucous  layers  of  the  epidermis  differs  greatly  in  different  parts  of 
the  body  ;  in  some  parts  of  the  palms  of  the  hands  and  soles  of  the 
feet,  the  horny  layer  is  very  thick,  and  here  we  often  find  that  the  cells 
which  lie  between  the  horny  and  mucous  layers  form  a  distinct,  narrow, 
transparent  zone,  called  the  stratum  lucidum. 

The  deeper  cells  of  the  mucous  layer  contain,  uniformly  in  the  ne- 
gro, and  occasionally  in  circumscribed  regions  in  white  men,  more  or 
less  brown  or  black  pigment.  The  epidermis  forms  in  but  few  regions 
of  the  body  a  layer  of  uniform  thickness,  since  the  corium  sends  up 
into  it,  at  varying  intervals,  simple  or  branching  variously  shaped 
fiapiilce,  the  valleys  between  which,  as  well  as  their  summits,  being 
covered  by  the  cells  of  the  mucous  layer.     If  we  imagine  a  section 


I  1 6  NORMAL   HISTOLOGY. 

made  through  the  skin  parallel  with  its  surface,  and  just  deep  enough 
to  touch  the  tops  of  the  papillae,  the  cells  of  the  mucous  layer  which 
lie  between  the  latter  would  appear,  on  looking  at  the  cut  surface,  to 
be  arranged  in  the  form  of  a  net-work  whose  meshes  are  filled  by  the 
papilla?  of  the  corium.  Hence  it  is  that  these  collections  of  cells  have 
received  the  name  rete  Malpighi.  The  corium  is  formed  of  interla- 
cing bundles  of  connective  tissue,  which  are  coarser  in  the  deeper, 
finer  in  the  more  superficial  portions,  where  they  extend  into  the  epi- 
dermis, forming  the  papilla?.  Elastic  fibres  are  present  in  considerable 
number,  and  in  the  interstices  between  the  fibres  lie  flattened,  spin- 
dle-shaped, branching  and  small  spheroidal  cells.  In  addition  to  these 
elements  we  find  muscular  tissue  in  the  corium  ;  striated  muscular 
fibres  occur  in  certain  parts  of  the  skin  of  the  face ;  and  smooth 
muscular  tissue,  aside  from  that  belonging  to  the  hair-follicle,  is  found 
about  the  sweat-glands,  in  the  skin  of  the  scrotum  and  penis,  and  in 
the  nipple  and  its  areola.  The  subcutaneous  connective  tissue  we 
have  already  studied  when  considering  the  connective  tissue  in  detail. 
In  some  parts  of  the  skin  the  texture  of  the  subcutaneous  tissue  is  so 
loose  that  the  corium  and  epidermis  can  be  readily  moved  to  and  fro 
upon  the  underlying  parts  or  pinched  up  in  folds  ;  in  others  its  fibres 
are  short  and  tense,  and  bind  the  corium  closely  to  the  parts  beneath. 
In  the  subcutaneous  tissue  of  most  parts  of  the  body,  greater  or 
smaller  deposits  of  fat  occur,  forming  the  panniculus  adiposus  ;  but  in 
the  subcutaneous  tissue  of  the  scrotum,  penis,  eyelids,  and  the  pinna 
of  the  ear,  fat  is  not  formed. 


The  Nail. 

We  recognize  in  the  hard  substance  of  the  nail,  which  corresponds 
to  the  horny  layer  of  the  epidermis,  a  body  and  a  root ;  the  former  lies 
upon  a  portion  of  the  somewhat  modified  corium  called  the  nail-bed, 
while  the  root  is  embedded  in  a  shallow  pocket  of  skin,  the  corium  of 
which  constitutes  the  matrix  of  the  nail.  The  corium  of  the  nail  does 
not  differ  essentially  from  that  of  the  skin  in  general;  it  is  intimately 
connected  with  the  periosteum  of  the  phalanx,  and  presents  longitu- 
dinal ridges  low  in  the  matrix,  higher  in  the  nail-bed,  which  are  cov- 
ered with  papillae.  These  are  bent  obliquely  forward,  and  are  more 
abundant  in  the  nail-bed  than  in  the  matrix.  Upon  and  between  the 
papillae  several  layers  of  variously  shaped  cells  lie,  corresponding  to 
the  mucous  layer  of  the  skin.  In  the  body  these  cells  pass  quite  ab- 
ruptly into  the  flat,  horny,  nucleated  cells  of  the  hard  substance  of  the 
nail  ;  in  the  matrix,  however,  the  transition  is  very  gradual,  and  it  is 
here  that  the  growth  of  the  nail  occurs.  The  lunula  is  a  portion  of 
the  nail  in  which  the  mucous  layer  is  very  thick,  as  it  is  in  all  parts  of 
the  root,  and  being  evenly  distributed  over  the  surface  of  the  papillae, 
does  not  permit  the  color  of  the  blood  in  the  capillaries  of  the  papil- 
lae to  be  seen  as  it  is  in  the  rest  of  the  nail-bed,  where  the  longitudinal 
ridges  are  higher  and  covered  by  fewer  of  the  translucent  cells  of  the 
mucous  layer. 


THE   SKIN.  117 


The  Hair. 


We  distinguish  in  the  hair:  the  shaft,  which  projects  above  the 
surface  of  the  skin  ;  the  root,  which  is  embedded  in  an  oblique  tubu- 
lar depression  called  the  follicle ;  and  the  bulb,  a  dilated  portion  at 
the  bottom  of  the  follicle  in  which  the  hair  ends.  The  follicle  some- 
times extends  into  the  subcutaneous  tissue,  sometimes  only  into  the 
corium,  and  its  walls  are  formed  in  the  first  place  by  a  sheath  of  con- 
nective tissue  continuous  with  the  corium,  in  which  numerous  blood- 
vessels ramify  ;  this  sheath  is  lined  by  a  thin  transparent  membrane 
called  the  vitreous  membrane.  Within  this  follicular  wall  lies  the 
root-sheath,  which  consists  of  two  layers  :  an  outer  thicker  layer, 
formed  by  the  dipping  down  into  the  follicle  of  the  cells  of  the  rete 
Malpighi,  and  hence  consisting  of  cylindrical  spheroidal  and  somewhat 
flattened  cells  ;  and  an  inner  layer  made  up  in  turn  of  an  external 
layer  called  Henle's  sheath,  in  which  the  cells  resemble  those  of  the 
horny  layer  of  the  epidermis,  and  are  closely  packed  together  to 
form  a  transparent  mass ;  and  an  internal  layer  called  Huxley's  sheath, 
whose  cells,  belonging  more  properly  to  the  hair  itself,  are  irregularly 
polygonal,  somewhat  flattened,  and  contain  an  elongated  nucleus. 
Both  at  the  opening  of  the  follicle  and  at  its  base,  the  layers  of  the 
root- sheath  become  indistinct,  merging  on  the  one  hand  into  the  cells 
of  the  epidermis,  and  on  the  other  into  those  of  the  hair-bulb.  At  the 
bottom  of  the  follicle  is  a  projection  from  the  connective  tissue  form- 
ing the  wall  of  the  follicle  in  the  form  of  a  papilla,  which  corresponds 
to  the  papillae  of  the  skin,  and  upon  which  the  hair-bulb  rests,  sur- 
rounding it  at  the  top  and  sides.  The  hair  is  produced  by  the  growth 
of  cells  about  the  papilla  ;  directly  covering  the  latter  are  cylindrical 
and  cuboidal  cells  corresponding  to  those  of  the  rete  Malpighi,  which 
gradually  become  changed  in  shape  and  more  or  less  horny,  and  form 
the  substance  of  the  hair-shaft.  In  the  shaft  we  recognize  three  por- 
tions :  1.  A  central  or  medullary,  composed  of  cuboidal  or  more  or 
less  flattened  cells,  not  infrequently  inclosing  between  them  tiny  bub- 
bles of  air  which  give  the  centre  of  the  hair  a  dark  appearance  by 
transmitted  light.  Outside  of  this  is  2,  the  cortical  portion,  mak- 
ing up  the  larger  part  of  the  bulk  of  the  shaft,  and  composed  of  tough, 
horny,  elongated,  flattened  cells  closely  packed  together,  and  contain- 
ing, except  in  colorless  hairs,  granules  of  variously  colored  pigment. 
In  that  portion  of  the  hair  which  lies  within  the  follicle,  and  between 
the  bulb  and  the  free  shaft,  we  find,  while  the  hair  is  growing,  that  the 
cells  of  the  cortical  layer  are  larger,  less  flattened  and  horny,  and,  as 
above  mentioned,  merge  into  the  large  cylindrical  and  spheroidal  cells 
of  the  bulb.  Finally,  the  shaft  is  covered,  3,  by  the  so-called  cuticula, 
consisting  of  thin,  rectangular,  non-nucleated,  scale-like  cells,  which 
lap  over  one  another  so  that  the  lower  cells,  i.  e.,  those  nearest  the 
root  of  the  hair,  cover  a  portion  of  the  cells  beyond,  and  these  free 
edges  often  project  slightly  from  the  surface  of  the  hair,  giving  it  a 
finely  serrated  appearance. 


Il8  NORMAL   HISTOLOGY. 


Sebaceous  Glands. 


These  are  racemose  glands  whose  excretory  ducts  are  lined  with 
cuboidal  or  somewhat  flattened  cells,  and  whose  alveoli,  bounded  by 
a  membrana  propria,  are  lined  with  transparent,  polygonal,  epithelial 
cells  ;  these  usually  contain,  in  addition  to  the  nucleus,  a  greater  or 
less  number  of  fat-droplets,  with  which,  also,  the  cavities  of  the  alveoli 
are  usually  more  or  less  completely  filled.  The  sebaceous  glands 
either  open  into  a  hair-follicle  near  the  surface  of  the  skin,  or  their 
excretory  ducts  are  pierced  near  the  surface  by  the  shaft  of  a  hair. 
The  hair-follicle,  as  above  mentioned,  is  placed  obliquely  in  the  skin, 
and  at  the  side  at  which  it  forms  an  oblique  angle  with  the  surface,  a 
bundle  of  smooth  muscle-cells  is  placed.  It  is  attached  to  the  con- 
nective-tissue sheath  of  the  follicle  in  its  lower  third,  and,  passing 
obliquely  upward,  is  inserted  into  the  upper  portion  of  the  corium  at 
some  distance  from  the  opening  of  the  follicle.  A  contraction  of  the 
muscle  thus  placed  will,  of  course,  draw  the  hair-follicle,  and  with 
it  the  shaft,  into  a  position  more  nearly  perpendicular  to  the  surface 
of  the  skin,  and  hence  it  is  called  the  erector  fiilce.  As  a  rule,  the 
sebaceous  follicle  lies  above  the  erector  muscle  in  the  angle  which  it 
forms  with  the  upper  portion  of  the  hair-follicle,  and  is  moved  with 
the  hair  when  the  muscle  contracts,  and  may  even  be  pressed  upon  by 
it  when,  as  is  frequently  the  case,  it  runs  closely  over  the  surface  of 
the  gland.  This  relation  of  the  erector  pike  muscle  to  the  sebaceous 
gland  is  not  without  significance  in  connection  with  the  discharge  of 
the  secretion  of  the  latter. 

Sweat-Glands. 

The  sweat-glands,  which  are  found  in  the  skin  of  almost  all  parts 
of  the  body,  although  much  more  abundant  in  some  than  in  others, 
are  tubular  glands,  the  tube  consisting  of  a  membrana  propria,  lined 
throughout  with  cuboidal  epithelium.  Its  lower  extremity,  coiled  into 
a  ball,  and  held  together  by  loose  connective  tissue,  lies  sometimes  in 
the  corium,  sometimes  in  the  subcutaneous  tissue.  The  upper  por- 
tion of  the  tube,  which  served  as  the  excretory  duct,  passes  to  the 
surface  of  the  skin,  often  taking  a  wavy  course  through  the  corium. 
It  pierces  the  epidermis  between  two  papillae,  and  here  the  walls  of  the 
duct  cease,  and  it  is  bordered  by  epidermis-cells  alone.  If  the  epi- 
dermis layer  is  thick,  as  in  the  palm,  etc.,  the  course  of  the  duct 
through  it  is  a  remarkably  winding  one.  An  abundant  capillary  net- 
work lies  in  the  loose  connective  tissue  of  the  gland-coil. 

Blood-vessels. 

The  arteries  of  the  skin,  which  enter  through  the  subcutaneous 
tissue,  give  off,  in  general,  three  sets  of  branches,  through  which  the 
blood  is  distributed  to  three  principal  sets  of  capillaries  :  first,  to  those 
which  supply  the  fat-tissue  ;  second,    to  those   which  ramify  in    the 


THE   SKIN.  IIQ 

sweat-glands ;  and  third,  to  those  which  supply  the  hair-follicles, 
sebaceous  glands,  and  the  papillae  of  the  coriura.  Each  papilla,  ex- 
cept when  containing  a  tactile  corpuscle,  is  furnished  with  a  capillary 
loop. 

Nerves. 

The  nerves  of  the  skin  ramify  in  the  subcutaneous  tissue,  and  a 
certain  number  of  them  terminate  here  in  the  so-called  Pacinian 
Bodies  ;  others  pass  into  the  corium,  where  they  form  plexuses,  varying 
in  character  in  different  parts  of  the  body ;  from  these  certain  medullated 
nerves  pass  into  the  papillae — those  which  contain  no  capillary  loops 
— and  terminate  in  the  tactile  corpuscles  ;  still  other  non-medullated 
nerves  pass  off  from  the  plexuses  and  enter  the  papillae,  or  pass  be- 
tween the  cells  of  the  rete  Malpighi,  but  their  mode  of  termination  is 
not  yet  definitely  ascertained.  The  Pacinian  bodies  are  found  in 
various  parts  of  the  human  body,  but  most  abundantly  along  the 
course  of  the  nerves  in  the  subcutaneous  tissue  of  the  palmar  and 
plantar  surfaces  of  the  fingers  and  toes.  They  are  oval  bodies,  readily 
visible  with  the  naked  eye,  consisting  of  a  number  of  concentrically- 
arranged — apparently  homogeneous — lamellae,  forming  a  series  of 
superimposed  capsules,  between  which  lie  endothelial  cells.  These 
lamellae,  thinner  and  more  closely  packed  near  the  centre  of  the  body, 
are  pierced  at  one  end  by  a  medullated  nerve ;  the  neurilemma  of 
the  nerve  seems  to  become  continuous  with  the  capsular  membranes, 
while  the  axis  cylinder,  with  its  medullary  sheath,  passes  inward.  The 
medullary  sheath  does  not  accompany  the  axis  cylinder  far,  but  gradu- 
ally dwindles  away;  the  naked  axis  cylinder  passes  along  in  a  cavity 
formed  by  the  innermost  capsule,  and  filled  with  a  granular  substance, 
and  terminates  near  the  opposite  end  of  the  body  in  a  dilated,  or 
branched  or  pointed  extremity ;  or  sometimes  passes  entirely  through 
the  body  and  becomes  continuous,  with  a  medullated  nerve-fibre  at 
the  other  end.  Capillary  blood-vessels  are  also  found  in  the  Pacinian 
bodies,  for  the  most  part  in  the  peripheral  zone. 

The  tactile  corpuscles,  called  also  Afeisszier's  corpuscles,  are  most 
abundant  at  the  finger-tips.  They  are  small  ellipsoidal  bodies,  with 
irregularly  transversely  striated  surfaces,  and  lie,  as  above  stated,  in 
the  papillae  of  the  skin.  They  consist  of  a  capsule  in  which  are  granu- 
lar material  whose  nature  is  undetermined,  elongated  nuclei  placed 
transversely  to  the  long  axis  of  the  capsule,  and  the  terminations  of 
the  nerves.  One  or  more  medullated  nerve-fibres  pass  to  the  lower 
extremity,  or  to  the  side  of  the  body,  and  sometimes  directly,  some- 
times after  winding  once  or  twice  around  it,  pass  beneath  the  capsule. 
Their  course  within  and  their  mode  of  termination  are  not  yet  defi- 
nitely ascertained. 

PRACTICAL    STUDY. 

Sections  of  Injected  Skin. — A  piece  of  skin  is  removed  from  an  in- 
jected leg  or  arm,  care  being  taken  to  include  the  subcutaneous  tissue 
to  a  considerable  depth  ;  it  is  stretched  flat  on  a  bit  of  cork  and  placed 


120  NORMAL   HISTOLOGY. 

in  a  mixture  of  equal  parts  of  one-half  per  cent,  chromic  acid  solution 
and  alcohol ;  after  ten  days  it  is  transferred  to  strong  alcohol.  When 
sufficiently  hard  it  is  embedded  in  wax  or  liver,  and  sections  made 
perpendicular  to  the  surface  ;  these  are  stained  lightly  with  hematoxy- 
lin, then  with  eosin,  and  mounted  in  balsam. 

Sections  of  Skin  of  Negro. — The  skin  is  hardened  in  the  chromic 
acid  and  alcohol  mixture,  the  sections  stained  with  picro-carmine, 
and  mounted  in  glycerine. 

Sections  of  the  Nail. — A  nail  should  be  separated  from  a  finger 
which  has  been  hardened  in  alcohol,  together  with  as  much  of  the 
connective  tissue  which  binds  it  to  the  bone  as  possible.  In  order  to 
render  the  soft  parts  more  consistent  and  more  nearly  like  the  body 
of  the  nail,  so  that  uniformly  thin  sections  can  be  made,  the  whole 
should  be  hardened  by  the  gum  method  ;  see  page  63.  Longitudinal 
and  transverse  sections  are  made  through  the  entire  nail,  and  stained 
first  very  deeply  with  a  strong  solution  of  aniline  green,  and  then  with 
eosin,  and  mounted  in  balsam. 

Sections  of  Hairs  from  Skin  of  Scalp. — A  piece  of  skin  from  the 
scalp  of  an  adult  is  stretched  on  cork  and  hardened  in  the  chromic 
acid  mixture.  Sections  are  made  as  nearly  as  possible  in  the  direc- 
tion of  the  hair-follicles,  so  as  to  include  the  entire  root  and  bulb,  and 
also  at  right  angles  to  them.  They  are  stained  with  aniline  green  and 
eosin,  and  mounted  in  balsam. 

Cells  of  the  Cuticnla  of  the  Hair. — These  are  readily  seen  by 
placing  a  hair  on  a  slide  in  a  drop  of  strong  sulphuric  acid  and  cover- 
ing ;  in  a  short  time  the  ceils  will  have  become  so  much  loosened  that 
if  the  cover  be  lightly  tapped  upon,  a  certain  number  of  them  will  float 
off  into  the  acid,  or  may  be  seen  bristling  out  from  all  sides  of  the 
shaft. 

Skin  from  the  Finger-tips. — This  is  prepared  like  the  injected 
skin,  and  will  show  the  thick  layer  of  epidermis-cells  with  the  stratum 
lucidum,  the  tortuous  course  of  the  sweat-gland  ducts  through  the 
epidermis,  the  tactile  corpuscles  and  numerous  sweat-glands,  and  very 
often  excellent  transverse  or  longitudinal  sections  of  the  Pacinian 
bodies  will  be  obtained,  if  the  subcutaneous  tissue  has  been  included 
to  a  considerable  depth  in  the  section.  The  Pacinian  bodies  are 
readily  studied  in  the  mesentery  of  the  young  cat,  where  they  are 
large  and  abundant ;  but  the  description  of  the  somewhat  complicated 
methods  to  which  we  must  have  recourse  in  order  to  completely 
demonstrate  their  structure,  would  carry  us  beyond  the  limits  of  these 
lessons. 


CHAPTER   XVIII. 

THE    EYE. 

The  organ  of  sight  is  composed  of  the  eyeball  and  various  acces- 
sory structures,  such  as  the  eyelids,  lachrymal  gland,  muscles,  etc. 
The  eyeball  is  composed,  in  the  first  place,  of  a  dense,  firm,  spheroidal, 
connective-tissue  envelope,  whose  anterior  transparent  portion,  the 
cornea,  is  more  convex  than  the  posterior  opaque  segment,  the  scle- 
rotic, and  differs  somewhat  from  it  in  structure  ;  the  sclerotic  is  pierced 
posteriorly  by  the  optic  nerve.  Within  the  sclerotic  lies  a  vascular 
tunic,  the  choroid,  formed  of  several  layers  of  tissue,  and  thrown  at 
the  surface,  not  far  from  the  sclero-corneal  junction,  into  numerous 
longitudinal  folds,  called  the  ciliary  processes.  An  extension  from  the 
ciliary  processes  passes  forward  into  the  iris,  which  is  a  perforated 
vascular  curtain  suspended  behind  the  cornea,  containing  radial  and 
circular  bundles  of  smooth  muscle-cells,  and  connected  peripherally 
near  the  sclero-corneal  junction,  with  a  connective-tissue  structure 
called  the  ligamentum pectinatum.  Passing  backward  from  the  liga- 
mentum  pectinatum,  between  the  ciliary  processes  and  the  sclera,  and 
attached  posteriorly  to  the  choroid,  is  a  muscle  having  the  form  of  a 
flattened  ring,  thickest  in  front,  called  the  ciliary  muscle.  The  direc- 
tion of  the  muscle-cells  in  the  ciliary  muscle,  which  are  of  the  smooth 
variety,  is  in  part  meridional  or  oblique,  in  part  circular.  The  ciliary 
processes  and  muscle  form  together  the  greater  part  of  a  structure 
known  as  the  ciliary  body.  The  retina,  the  innermost  of  the  layers 
forming  the  wall  of  the  eyeball,  spreads  out  from  the  point  of  entrance 
of  the  optic  nerve  over  the  inner  surface  of  the  choroid.  At  about  a 
third  of  the  distance  back  from  the  front  of  the  eye,  the  nerve  ele- 
ments of  the  retina  cease  in  a  wavy  line,  called  the  ora  serrata  ;  cer- 
tain of  the  connective  tissue  and  cellular  elements  continue,  however, 
somewhat  modified  in  character,  over  the  ciliary  processes,  under  the 
name  of  the  pars  ciliaris  retina.  The  crystalline  lens  is  suspended 
close  behind  the  iris  by  a  firm,  delicate,  fibrillated  membrane  called 
the  suspensory  ligament,  which  is  attached  on  the  one  hand  to  a  mem- 
brane covering  the  ciliary  processes,  and  on  the  other  to  the  capsule 
of  the  lens.  The  cavity  of  the  eyeball  is  divided  by  the  lens  and  its 
suspensory  ligament  into  two  chambers,  the  anterior  and  smaller  of 
which  is  filled  with  a  homogeneous  fluid,  the  aqueous  humor  ;  the  pos- 
terior with  a  gelatinous  substance,  the  vitreous  humor,  which  presents 
an  ill-defined,  lamellar  structure,  and  sometimes  contains  a  variable 
number  of  ill-defined,  more  or  less  granular  cells.     The  vitreous  is 


122  NORMAL   HISTOLOGY. 

surrounded  by  a  delicate  membrane,  called  the  hyaloid  membrane, 
which  is  closely  connected  with  the  lining  membrane  of  the  retina, 
and  is  hardly  to  be  differentiated  from  it. 

Having  thus  briefly  described  the  general  structure  of  the  eye,  it 
remains  for  us  to  consider  some  of  its  parts  somewhat  more  in  detail ; 
the  limits  of  the  course  will  not  permit  us,  however,  to  make  an  ex- 
tended study  of  all  or  even  any  of  the  structures  in  the  eye  ;  we  shall 
be  obliged  to  confine  ourselves  to  the  more  marked  features  of  the 
cornea  and  sclera,  the  posterior  portions  of  the  choroid  and  retina,  and 
the  crystalline  lens. 

Sclera. 

The  sclera  is  composed  of  very  closely  interwoven  connective-tis- 
sue fibres,  with  fine  elastic  fibres,  the  latter  most  abundant  near  the 
inner  surface.  Between  the  fibres,  which  have  little  regularity  in  their 
arrangement,  lie  flat  connective-tissue  cells,  a  certain  number  of  which 
frequently  contain  pigment-granules.  On  the  external  surface,  the 
sclera  sends  off  delicate  fibres  ;  anteriorly  into  the  subconjunctival 
tissue  ;  while  posteriorly,  behind  the  muscle  tendons,  they  join  to  form 
the  wall  of  a  lymph-sac,  called  the  capsule  of  Tenon.  On  the  inner 
surface  certain  fibres  pass  directly  over  into  the  choroid,  others  form 
the  outer  wall  of  a  lymph-sac  between  the  sclera  and  choroid,  and 
called,  on  account  of  its  yellow  color,  the  lamina  f us ca  ;  it  resembles 
in  structure  the  outer  layers  of  the  choroid,  presently  to  be  described 
as  the  membrana  supra-choroidea,  of  which  it  is  indeed  a  part.  In  man 
and  many  animals,  the  opening  in  the  posterior  segment  of  the  sclera, 
through  which  the  optic  nerve  passes,  is  crossed  by  a  net-work  of  con- 
nective-tissue fibres  ;  these  pass  in  from  the  sclera  on  all  sides,  and  sur- 
round the  delicate  bundles  of  nerve-fibres  of  the  opticus,  forming  the 
lamina  cribrosa. 

Cornea. 

The  cornea  is  directly  continuous  at  the  periphery  with  the  sclera, 
but  differs  from  it  in  structure  in  many  particulars,  among  the  more 
prominent  of  which  are  :  the  more  regular  lamellar  arrangement  of  its 
connective-tissue  basement  substance ;  the  greater  abundance  and 
peculiar  form  of  its  cellular  elements  ;  and  the  free  anterior  and  pos- 
terior surface  covered  with  cells.  In  a  thin  section  of  the  cornea,  per- 
pendicular to  its  surface,  we  recognize,  passing  from  before  backward, 
five  layers :  i.  a  stratified  layer  of  epithelial  cells — the  anterior  corneal 
epithelium — consisting  of  cells  resembling  in  general  form  and  arrange- 
ment those  of  the  epidermis ;  that  is,  we  have  in  the  deepest  layer 
cylindrical  cells,  passing  over  into  spheroidal,  and  these  into  flattened 
cells  at  the  surface  ;  2.  The  anterior  epithelium  rests  on  a  dense  trans- 
parent membrane,  called  the  anterior  basal  membrane,  or  lamina  elas- 
tica  anterior,  which  is  continuous  within  with  the  connective-tissue 
basement  substance  proper  of  the  cornea,  and  is  composed  of  closely 
packed  fibrillae  ;  3.  The  body  of  the  cornea — substantia  propria  comece 
— is  composed  of  connective  tissue  whose  characteristics  we  have  al- 
ready studied,  page  25  et  seq.;  4.   Lying  closely  upon  the  posterior 


THE   EYE.  123 

surface  of  the  last  layer,  is  a  thin,  apparently  structureless  membrane 
— the  membrane  of  Descemet,  lamina  elastica  posterior — upon  which 
lies  :  5.  A  single  layer  of  flattened  cells,  usually  called  the  endothelium 
of  Descemet.  The  cornea,  except  at  its  extreme  periphery,  contains 
no  blood-vessels.  Nerves,  on  the  other  hand,  are  very  abundant. 
These,  in  larger  and  smaller  trunks,  enter  the  cornea  at  the  periphery, 
and  dividing  and  subdividing,  presently  loose  the  medullary  sheath 
and  break  up  into  bundles  of  extremely  delicate  fibres  •  these,  passing 
toward  the  anterior  surface,  separate  and  form  extraordinarily  delicate 
and  intricate  plexuses,  while  here  and  there  somewhat  enlarged  nodal 
points  are  seen  along  the  fibres,  from  which  the  often-beaded  fibrils 
pass  off  in  all  directions.  The  exact  mode  of  termination  of  the 
nerve-fibrils  in  the  cornea  is  not  definitely  ascertained ;  certain  of  them, 
however,  seem  to  pass  between  the  anterior  epithelial  cells,  and  are  be- 
lieved by  some  investigators  to  terminate  in  free  extremities  at  the 
surface. 

Choroid. 

In  the  posterior  portion  of  the  eye,  that  lying  behind  the  ora  ser- 
rata,  the  choroid  presents  four  layers,  which  although  intimately  con- 
nected and  presenting  no  sharp  line  of  separation,  may  yet  be  more  or 
less  completely  separated  by  a  careful  dissection.  Directly  beneath 
the  lamina  fusca  of  the  sclera,  and  forming  the  inner  wall  of  the  above- 
mentioned  lymph-sac,  is  the  outermost  layer,  called  the  lamina  supra- 
choroidea  ;  it  is  composed  of  a  series  of  super-imposed  connective-tissue 
membranes,  containing  a  net-work  of  delicate  elastic  fibres,  between 
which  lie  numerous  flattened,  irregular-shaped,  often-branched,  pig- 
mented cells.  Within  the  lamina  supra-choroidea  lies  a  layer  containing 
the  larger  vessels,  arteries  and  veins  of  the  choroid,  and  called  the  exter- 
nal vascular  layer,  or  the  layer  ofHaller.  This  layer  is  composed  of  a 
ground-work  similar  in  structure  to  the  supra-choroidea,  in  which  the 
vessels  are  embedded,  the  arteries  being  often  accompanied  by  smooth 
muscle-cells.  The  inner  vascular  layer,  called  the  chorio-capillaris, 
follows  next,  and  consists  almost  entirely  of  a  very  dense  net-work  of 
broad  capillary  blood-vessels.  Finally,  the  choroid  is  limited  within 
by  an  extremely  delicate,  finely  striated  membrane,  called  the  mem- 
brane of '  Bruch. 

Retina. 

Of  all  the  animal  structures  the  retina  is  one  of  the  most  delicate 
complicated,  and  difficult  of  study,  and  we  can  do  little  here  than  indi- 
cate briefly  the  general  grouping  of  its  elements.  It  consists  of  a 
connective-tissue  frame-work  by  which  the  nerve-elements  are  sup- 
ported, and  with  which  they  are  most  intimately  associated ;  and  in 
some  cases  it  is  as  yet  impossible  to  say  to  which  variety  of  tissue  a 
given  element  belongs.  We  distinguish,  in  typical  parts,  ten  layers ; 
commencing  from  within,  they  may  be  enumerated  as  follows : 

1.  Membrana  limitans  interna. 

2.  Layer  of  optic  nerve-fibres. 

3.  Layer  of  ganglion-cells. 


124  NORMAL   HISTOLOGY. 

4.  Internal  molecular  layer. 

5.  Internal  granular  layer. 

6.  External  molecular  layer. 

7.  External  granular  layer. 

8.  Membrana  limitans  externa. 

9.  Layer  of  rods  and  cones. 
10.  Pigment  layer. 

The  limiting  membranes,  the  outer  of  which  is  perforated  by 
numerous  openings,  are  delicate  and  homogeneous,  and  present  no 
points  of  special  interest  for  our  consideration.  In  the  layer  of  optic 
nerve-fibres,  the  fibres  run  for  the  most  part  in  a  horizontal  direction 
and  pass  finally  into  the  next  layer  to  join  the  ganglion-cells;  these, 
having  the  general  characters  of  branching  nerve-cells,  send  numerous 
processes  into  the  internal  molecular  layer,  where  they  break  up  into 
a  delicate  fibrillar  net-work,  associated  with  the  connective-tissue 
frame-work.  In  the  internal  granular  layer,  the  so-called  granules  consist 
of  small  nerve-cells  and  large  nuclei,  belonging  to  the  connective-tissue 
frame-work  of  the  retina.  In  the  external  molecular  layer  again,  we 
have  a  delicate  net-work  of  nerve-fibrils  mingled  with  connective  tissue. 
The  granules  of  the  external  granular  layer  are  nerve-elements  exclu- 
sively, and  are  intimately  connected  by  processes  which  pass  through 
the  openings  in  the  internal  limiting  membrane,  with  the  rods  and 
cones.  Of  the  latter  structures,  which  within  the  limits  of  these  les- 
sons we  cannot  even  in  a  general  way  adequately  describe,  the  rods 
are  the  longer,  are  somewhat  conical  at  the  outer  extremity  where  they 
join  the  nerve-elements  of  the  outer  granular  layer;  the  cones  are 
shorter,  are  connected  also  with  the  nerve-elements  within  and  termi- 
nate externally  in  pointed  extremities.  The  connective-tissue  elements 
of  the  retina  consist,  in  certain  layers,  of  broad,  irregular  radial  fibres 
forming  frequent  inosculations,  and  in  the  unequally  thick  molecular 
layers  of  a  delicate  reticulum  within  which  the  nerve-fibrils  ramify. 

The  pigment  epithelium,  in  the  form  of  a  single  layer  of  flat  poly- 
gonal cells,  which  we  have  already  studied,  rests  upon  the  outer  ends 
of  the  rods,  and  is  usually  left  upon  the  choroid  when  the  retina  is 
separated  from  the  latter. 

The  Lens. 

The  lens  is  a  transparent  double  convex  body,  of  sufficient  firm- 
ness to  maintain  its  form  when  removed  from  the  eye,  and  is  enclosed 
in  a  homogeneous  elastic  capsule  which  is  thicker  on  the  anterior 
than  on  the  posterior  surface.  To  the  peripheral  zone  of  the  capsule 
on  both  anterior  and  posterior  surfaces  the  suspensory  ligament  is 
firmly  attached.  The  body  of  the  lens,  although  perfectly  transparent, 
is  by  no  means  structureless.  Behind  the  anterior  wall  of  the  capsule 
Lies  a  single  layer  of  flattened  polygonal  cells,  which  at  the  equator 
gradually  lengthen  out  to  form  very  much  elongated  band-like  nuclea- 
ted cells,  or  le?is-fibres,  which  running  meridionally  make  up  the  greater 
part  of  the  body  of  the  lens.  The  lens-fibres  have,  on  transverse  sec- 
tion, a  flattened  hexagonal  form  ;  they  are  joined  intimately  to    one 


THE   EYE.  125 

another  by  their  narrower  sides,  presenting  the  broader  sides  to  the 
surfaces  of  the  lens,  so  that  under  certain  circumstances  they  can  be 
peeled  off  in  layers.  The  course  of  the  fibres  in  the  lens  is  somewhat 
complicated,  but  it  may  be  said  in  general  that  they  run  meridionally 
from  one-half  of  the  lens,  in  broad  sweeps  over  into  the  other  ;  inas- 
much, however,  as  the  individual  fibres  are  not  long  enough  to  reach 
the  entire  distance  from  one  pole  of  the  lens  around  to  the  other, 
they  commence  along  certain  definite  lines  at  varying  distances  from 
the  poles ;  and  the  farther  from  one  pole  one  end  of  the  fibre  is,  the 
nearer  to  the  other  will  its  other  end  lie.  In  the  young  human  lens, 
the  lines  from  which  the  fibres  start  may  be  seen  on  the  anterior  and 
posterior  surfaces,  under  certain  circumstances,  in  the  form  of  a  three- 
rayed  star  ;  in  the  adult  the  rays  usually  part  at  the  ends,  giving  rise 
to  complicated  stellate  figures. 


The  Eyelids. 

These  are  formed  in  general  by  a  plate  of  connective  tissue  which 
toward  the  free  border  is  very  dense  and  firm,  and  called  the  tarsal 
cartilage  or  tarsus  ;  they  are  covered  on  the  outside  by  skin,  on  the 
inside  by  the  conjunctival  mucous  membrane  ;  between  the  tarsus  and 
the  skin  lie  the  bundles  of  the  musculus  orbicularis.  The  tarsus, 
which  is  in  no  sense  cartilage,  is  a  plate  formed  of  very  dense  and 
firm  fibrillar  connective  tissue  containing  ordinary  flattened  connective- 
tissue  cells,  and  is  closely  connected  within  with  the  palpebral  con- 
junctiva. Embedded  within  the  tarsus  lie  the  Meibomian  glands, 
thirty  to  forty  in  number  in  each  lid.  They  consist  of  numerous  vesi- 
cular alveoli,  lined  with  short  cylindrical  cells,  arranged  along  and 
opening  into  long  excretory  ducts  which  are  lined  with  laminated 
epithelium,  and  open  on  the  inner  border  of  the  edge  of  the  lids. 
They  are  somewhat  modified  sebaceous  glands,  but  unlike  most  seba- 
ceous glands  are  not  connected  with  hair-follicles.  The  skin  of  the 
eyelids  is  somewhat  thinner  than  that  of  the  face,  is  beset  with  deli- 
cate hairs,  and  supplied  with  sweat-glands  and  sebaceous  follicles.  It 
passes  over  on  to  the  edge  of  the  lids,  at  the  inner  border  of  which  it 
becomes  continuous  with  the  mucous  membrane.  The  eyelashes  are 
inserted  obliquely  into  the  edge  of  the  lid  in  from  two  to  four  rows  ; 
the  follicles,  which  are  quite  deep,  are  furnished  with  sebaceous  glands. 
The  conjunctival  mucous  membrane  of  the  lids  consists  of  a  basis  sub- 
stance of  loose  fibrillar  connective  tissue  containing  a  few  elastic 
fibres  and  numerous  small  spheroidal  and  branching  cells.  The  epi- 
thelium is  laminated,  consisting  of  a  deep  layer  of  cylindrical  cells, 
then  more  superficially  of  smaller  spheroidal  or  oblong  cells,  and 
finally  of  flattened  cells  on  the  surface.  Small  racemose  glands,  called 
accessory  tear-glands,  are  often  seen  opening  on  the  surface  of  the 
mucous  membrane.  In  addition  to  the  striated  muscular  bundles  of 
the  orbicularis,  smooth  muscle-cells  forming  a  membranous  layer  occur 
beneath  the  conjunctiva  on  the  orbital  portion  of  the  lid. 


I2D  NORMAL   HISTOLOGY. 


PRACTICAL    STUDY. 


General  Dissection  of  the  Eye. — For  this  purpose  a  large  eye, 
like  that  of  the  sheep  or  ox,  is  preferable  ;  it  should  be  opened  by  a 
short  incision  through  the  sclerotic,  so  that  the  fluid  can  readily  come 
into  contact  with  the  parts  within,  and  placed  in  M  tiller's  fluid  ;  after 
two  weeks  the  dissection  may  be  made.  The  eye  should  be  divided 
with  a  sharp  razor  into  lateral  halves.  The  cut  surface  shows  at  once 
the  general  relations  of  the  parts  ;  cornea,  iris,  lens,  ciliary  body, 
vitreous,  retina,  choroid  and  sclera  are  seen  in  approximately  normal 
relations  to  one  another.  If  the  vitreous  body  be  .,ow  carefully  re- 
moved from  one  of  the  halves,  the  retina  and  ciliary  body  come 
more  fully  into  view  ;  and  the  suspensory  ligament  is  readily  seen. 
As  the  suspensory  ligament  approaches  the  edge  of  the  lens  it  divides 
into  two  layers  which  pass,  one  to  the  anterior,  the  other  to  the  poste- 
rior surface  of  that  body,  leaving  a  slit-like  opening  between  them  ; 
this  is  called  the  canal  of  Petit,  and  may  be  readily  seen  by  slightly 
pulling  the  edge  of  the  lens  backward  and  inward,  when  the  layers 
will  separate.  Now  seizing  the  half  of  the  lens  with  a  pair  of  forceps 
and  carefully  drawing  it  downward  and  backward,  awTay  from  the  iris, 
the  suspensory  ligament,  in  the  form  of  a  folded  fringe-like  membrane, 
may  be  torn  away  from  its  attachment  to  the  hyaloid  membrane  ;  a 
portion  of  it  is  snipped  off  close  to  the  lens,  stained  in  hematoxylin 
and  mounted  in  glycerin.  Bits  of  the  lens  capsule,  anterior  and  pos- 
terior surfaces,  may  be  peeled  off  and  examined  in  glycerine  ;  and  a 
fragment  of  the  lens-tissue  may  be  teased  on  a  slide,  stained  with  eosin 
and  also  mounted  in  glycerine.  After  the  removal  of  the  lens,  the 
general  form  and  attachment  of  the  iris  are  readily  seen.  In  the  same 
half  of  the  eye  the  layers  of  the  choroid  may  be  demonstrated.  The 
retina  is  pulled  off  and  the  pigmented  cells,  which  are  apt  to  adhere 
to  the  inner  surface  of  the  choroid,  are  brushed  away  under  water 
with  a  fine  pencil.  The  eye  is  now  pinned,  with  the  flat  side  down, 
on  to  a  cork  which  has  been  attached  to  a  bit  of  sheet  lead  and  sunk 
in  a  shallow  dish  of  water.  A  window  is  cut  through  the  sclera,  in- 
eluding  the  sclero-corneal  junction  ;  the  bit  of  sclera  having  been 
removed,  care  being  taken  not  to  bring  away  the  ciliary  body,  the 
membranasupra-choroideawill  be  seen  as  a  brown  loose  tissue  floating 
at  the  surface  of  the  choroid.  Bits  of  this  are  seized  with  the  for- 
ceps upon  the  ciliary  body,  and  drawn  carefully  backward  so  as  to 
separate  them  from  the  parts  beneath  ;  with  care,  very  thin  bits  of 
considerable  size  may  be  separated.  They  should  be  floated  without 
wrinkles  or  folds  on  to  a  slide  immersed  in  water,  then  stained  on  the 
slide  with  hematoxylin  and  mounted  in  glycerine.  The  floating  shreds 
of  the  supra-choroidea  which  remain  after  suitable  specimens  have 
been  obtained,  should  now  be  pulled  from  the  eye,  when  the  vascular 
layer  of  Haller  will  come  into  view.  The  separation  of  the  three 
remaining  layers  is  not  easy  under  the  most  favorable  conditions,  and 
is  especially  difficult  in  the  eye  of  the  ox  and  sheep,  where  the  layers 
are  rendered  more  complicated  and  difficult  of  separation  by  the  pres- 


THE   EYE.  127 

ence  of  a  mass  of  interlacing  fibres.  Haller's  layer,  however,  and 
the  membrana  chorio-capillaris  may  be,  with  care,  stripped  off  in 
pieces  sufficiently  thin  for  demonstration ;  sometimes  a  fragment  will 
be  obtained,  which,  especially  at  the  edges,  will  show  both  of  the 
vascular  layer  at  once.  They  are  stained  with  hematoxylin  and 
mounted  in  glycerine. 

Cornea  and  Sclera. — In  a  fresh  eye  from  the  rabbit  or  dog,  a 
short  incision  is  made  through  the  sclera;  it  is  placed  in  Muller's 
fluid,  and,  after  three  weeks,  washed,  and  the  hardening  completed  in 
alcohol.  The  cornea  should  now  be  excised  close  to  the  sclero-cor- 
neal  junction,  embedded  in  wax,  and  thin  sections  made  perpendicu- 
lar to  the  surface,  including  the  entire  thickness,  stained  with  hsema- 
toxylin and  eosin,  and  mounted  in  glycerine. 

Sclero-comeal  Junction. — The  cornea  and  sclera,  in  their  relations 
to  one  another,  and  the  ciliary  bodies  and  iris  may  be  examined  in  a 
specimen  prepared  as  follows  :  An  eye — that  of  the  pig  is  best,  if  a 
fresh  human  eye  cannot  be  procured — is  placed,  after  making  a  short 
incision  in  the  sclera,  in  Midler's  fluid,  where  it  remains  for  two 
weeks,  the  fluid  being  changed  once  or  twice  ;  it  is  then  soaked  for 
an  hour  or  two  in  water,  and  the  hardening  completed  in  dilute  and 
strong  alcohol.  A  bit  is  excised,  including  the  sclero-comeal  junc- 
tion and  adjacent  parts,  and  embedded  in  cacao  butter  by  the  method 
given  on  page  93.  Thin  sections  are  made  perpendicular  to  the  sur- 
face of  the  eye,  and  after  removing  the  cacao  butter,  stained  in  hema- 
toxylin and  eosin,  and  mounted  in  balsam. 

Nerves  of  the  Cornea. — The  following  is  Ranvier's  method  for 
their  demonstration  :  The  cornea  of  a  recently-killed  rabbit  is  placed 
for  five  minutes  in  a  small  quantity  of  freshly-prepared  and  filtered  lem- 
on-juice ;  it  is  then  placed  in  a  few  cubic  centimetres  of  one  per  cent, 
solution  of  chloride  of  gold,  in  which  it  remains  fifteen  or  twenty  min- 
utes ;  it  is  then  washed  and  placed  in  a  small  bottle,  in  which  are 
twenty-five  to  thirty  cubic  centimetres  of  distilled  water,  to  which  a 
drop  or  two  of  acetic  acid  has  been  added.  In  twenty-four  or  forty- 
eight  hours,  under  the  action  of  the  light,  the  gold  will  be  reduced, 
and  the  cornea  have  a  violet  color.  Sections  are  made  parallel  with 
the  surface  ;  the  portions  directly  beneath  the  anterior  epithelium  are 
the  most  desirable  for  study,  since  here  the  delicate  nerve  net-work  is 
most  abundant.  Sections  may  also  be  made  perpendicular  to  the 
surface,  especial  care  being  taken  to  preserve  the  anterior  epithelium 
undisturbed. 

Sections  of  the  Lens. — The  eye  of  a  rabbit  or  pig,  in  which  the 
sclera  has  been  opened,  should  be  placed  in  Muller's  fluid ;  after 
about  a  fortnight  the  lens  is  removed,  care  being  taken  not  to  rupture 
the  capsule,  kept  for  a  few  days  in  alcohol,  and  embedded  in  wax. 
Thin  sections  are  made  from  before  backward,  through  the  centre, 
stained  with  hsematoxylin,  and  mounted  in  balsam. 

Transverse  Sections  of  the  Retina. — In  a  fresh  human  or  pig's 
eye,  one  or  two  small  openings  should  be  made  through  the  sclera,  so 
that  the  fluid  can  readily  enter,  and  placed  in  Muller's  fluid ;  after  a 
week  the  eye  may  be  cut  across  just  behind  the  sclero-corneal  June- 


123  NORMAL   HISTOLOGY. 

tion,  and  the  posterior  segment  placed  in  fresh  Miiller's  fluid;  after 
another  week  the  eye  is  transferred  for  twenty-four  hours  to  dilute, 
then  to  strong  alcohol.  After  remaining  for  a  day  or  two  in  strong 
alcohol,  a  small  bit  is  cut  from  the  posterior  portion  of  the  retina, 
about  midway  between  the  optic-nerve  entrance  and  the  pars-ciliaris 
retinae,  and  embedded  in  cacao  butter  ;  thin  transverse  sections  are 
then  made  ;  the  cacao  butter  dissolved  out  in  the  usual  way :  the  sec- 
tions stained  with  hematoxylin  and  eosin,  and  one  or  two  mounted  in 
glycerine,  one  or  two  in  balsam.  By  this  method  the  general  arrange- 
ment of  the  layers  is  shown,  but  many  of  the  finer  structures,  espe- 
cially the  rods  and  cones,  are  much  altered ;  and  for  the  study  of 
these,  we  have  recourse  to  osmic  acid  preparations. 

Isolated  Retiiial  Elemefits. — A  small  fragment  of  the  perfectly 
fresh  retina  of  man  or  the  pig  is  placed  in  one  per  cent,  solution  of 
osmic  acid ;  in  forty-eight  hours  it  is  removed,  and  allowed  to  soak  in 
water  for  a  few  hours  ;  it  is  then  placed  in  a  mixture  of  equal  parts  of 
glycerine,  alcohol,  and  water,  where  it  remains  for  a  week  or  ten 
days ;  small  fragments  are  then  either  teased  carefully  on  a  slide  in 
glycerine,  and  covered,  the  specimen  being  protected  from  pressure 
by  a  hair ;  or,  what  is  better,  a  small  fragment  is  put  on  a  slide  in  a 
drop  of  glycerine,  and  a  large  cover-glass,  on  the  under  side  of  which, 
at  the  corners,  tiny  fragments  of  white  wax  have  been  placed,  is  in- 
verted over  it ;  and  the  cover,  just  above  the  specimen,  is  gently  and 
repeatedly  tapped  upon  with  a  needle  ;  the  movement  thus  occa- 
sioned in  the  glycerine  will,  after  a  time,  cause  the  retina  to  fall  apart, 
and  become  separated  into  exceedingly  small  fragments.  This  latter 
method  is  known  as  Rindfleisctis  Dissociation  Method,  and  may  be 
employed  to  advantage  in  the  dissociation  of  many  kinds  of  tissue — 
that  of  the  central  nervous  system,  for  example — when  the  use  of 
needles  would  be  apt  to  cause  injury  to  the  isolated  elements. 


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